diff --git a/browser/components/shell/ShellService.sys.mjs b/browser/components/shell/ShellService.sys.mjs
index ee2cdd8fc7ba..d047c15358fe 100644
--- a/browser/components/shell/ShellService.sys.mjs
+++ b/browser/components/shell/ShellService.sys.mjs
@@ -12,6 +12,8 @@ ChromeUtils.defineESModuleGetters(lazy, {
   ASRouter: "resource:///modules/asrouter/ASRouter.sys.mjs",
   ScheduledTask: "resource://gre/modules/ScheduledTask.sys.mjs",
   Subprocess: "resource://gre/modules/Subprocess.sys.mjs",
+  WindowsSetDefaultRedirect:
+    "moz-src:///browser/components/shell/WindowsSetDefaultRedirect.sys.mjs",
   WindowsVersionInfo:
     "resource://gre/modules/components-utils/WindowsVersionInfo.sys.mjs",
 });
@@ -422,7 +424,36 @@ let ShellServiceInternal = {
     );
   },
 
-  async setAsDefaultPDFHandler(onlyIfKnownBrowser = false) {
+  /**
+   * Returns the on-disk nsIFile for a PDF bundled under the browser directory
+   * in NS_GRE_DIR.
+   *
+   * @param {string} aLeafName - The bundled PDF's file name, e.g.
+   * "confused_fox.pdf".
+   * @returns {nsIFile} The bundled file (which may not exist on disk).
+   */
+  getBundledPdfFile(aLeafName) {
+    const file = Services.dirsvc.get("GreD", Ci.nsIFile);
+    file.append("browser");
+    file.append(aLeafName);
+    return file;
+  },
+
+  /**
+   * Set Firefox as the Windows default PDF handler.
+   *
+   * @param {boolean} [onlyIfKnownBrowser] - When true, only proceed if the
+   * current default PDF handler is a known browser.
+   * @param {boolean} [openInFirefox] - Only meaningful on the "Open with"
+   * picker code path. After the user picks Firefox, the OS relaunches Firefox
+   * with the bundled stub PDF; this flag decides whether we then open a PDF in
+   * a new tab (true), to land the user in Firefox, or silently absorb that
+   * relaunch (false).
+   */
+  async setAsDefaultPDFHandler(
+    onlyIfKnownBrowser = false,
+    openInFirefox = false
+  ) {
     if (AppConstants.platform != "win") {
       throw new Error("Windows-only");
     }
@@ -453,10 +484,6 @@ let ShellServiceInternal = {
       );
     }
 
-    const winShell = this.shellService.QueryInterface(
-      Ci.nsIWindowsShellService
-    );
-
     // Optional second attempt via the undocumented IOpenWithLauncher API,
     // which surfaces the OS "Open with" picker so the user can pick Firefox
     // themselves. Gated by a pref so it can be remotely disabled if it
@@ -469,10 +496,27 @@ let ShellServiceInternal = {
       )
     ) {
       method = "open_with";
+      const openWithArg = this.getBundledPdfFile("confused_fox.pdf").path;
+      // Arm the round-trip: the OS hands `openWithArg` back to Firefox if the user
+      // selects us. We redirect that launch to the bundled PDF); otherwise overrideUri
+      // is null and the launch is suppressed.
+      const overrideUri = openInFirefox
+        ? Services.io.newFileURI(this.getBundledPdfFile("blank.pdf")).spec
+        : null;
+      lazy.WindowsSetDefaultRedirect.arm(
+        openWithArg,
+        overrideUri,
+        lazy.WindowsSetDefaultRedirect.TYPE.FILE
+      );
+
       try {
-        winShell.launchOpenWithDefaultPickerForFileType(".pdf");
+        const flags = this._isWindows11()
+          ? Ci.nsIWindowsShellService.OPEN_WITH_SET_HANDLER
+          : Ci.nsIWindowsShellService.OPEN_WITH_SET_HANDLER_WIN10;
+        this.shellService.launchSetDefaultAppPicker(openWithArg, flags);
         success = true;
       } catch (e) {
+        lazy.WindowsSetDefaultRedirect.clear();
         // The picker API itself failed (e.g. COM error). Fall through to the
         // modern settings dialog rather than leaving the user without any
         // default-handler UI.
@@ -487,7 +531,7 @@ let ShellServiceInternal = {
     if (!success && this._isWindows11()) {
       method = "settings";
       try {
-        winShell.launchModernSettingsDialogDefaultApps();
+        this.shellService.launchModernSettingsDialogDefaultApps();
         Glean.browser.setDefaultPdfHandlerModernSettingsResult.Success.add(1);
         success = true;
       } catch (e) {
@@ -524,9 +568,7 @@ let ShellServiceInternal = {
    */
   isDefaultHandlerFor(aFileExtensionOrProtocol) {
     if (AppConstants.platform == "win") {
-      return this.shellService
-        .QueryInterface(Ci.nsIWindowsShellService)
-        .isDefaultHandlerFor(aFileExtensionOrProtocol);
+      return this.shellService.isDefaultHandlerFor(aFileExtensionOrProtocol);
     }
     return false;
   },
diff --git a/browser/components/shell/WindowsSetDefaultAppCmdHandler.sys.mjs b/browser/components/shell/WindowsSetDefaultAppCmdHandler.sys.mjs
new file mode 100644
index 000000000000..88de305e30d8
--- /dev/null
+++ b/browser/components/shell/WindowsSetDefaultAppCmdHandler.sys.mjs
@@ -0,0 +1,97 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * Command-line handler for the Windows IOpenWithLauncher round-trip.
+ *
+ * When the ShellService.setAsDefault{PDF,Protocol}Handler launches the windows only
+ * "Open with" picker, it hands it a bundled stub file path for file-type
+ * defaults, or a URL for protocol defaults, and stashes a one-shot
+ * { openWithArg, overrideUri } redirect.
+ *
+ * If the user picks Firefox, Windows invokes Firefox with
+ * `-osint -url <openWithArg>`. This handler runs before BrowserContentHandler and
+ * intercepts the launch: it asks ShellService whether the -url
+ * value matches the pending openWithArg and, if so, suppresses the
+ * open and optionally redirects to the stashed overrideUri so the user lands
+ * somewhere meaningful in Firefox.
+ */
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  BrowserWindowTracker: "resource:///modules/BrowserWindowTracker.sys.mjs",
+  WindowsSetDefaultRedirect:
+    "moz-src:///browser/components/shell/WindowsSetDefaultRedirect.sys.mjs",
+});
+
+ChromeUtils.defineLazyGetter(lazy, "logConsole", () => {
+  return console.createInstance({
+    prefix: "WindowsSetDefaultAppCmdHandler",
+    maxLogLevel: "Warn",
+  });
+});
+
+export class CommandLineHandler {
+  static classID = Components.ID("{da7de528-7a15-452e-b5a7-521099997ca1}");
+  static contractID = "@mozilla.org/browser/windows-default-clh;1";
+
+  QueryInterface = ChromeUtils.generateQI([Ci.nsICommandLineHandler]);
+
+  handle(aCmdLine) {
+    if (aCmdLine.findFlag("osint", false) < 0) {
+      return;
+    }
+
+    const urlIdx = aCmdLine.findFlag("url", false);
+    if (urlIdx < 0) {
+      return;
+    }
+
+    const cmdArg = aCmdLine.getArgument(urlIdx + 1);
+
+    // null: not our openWithArg, leave -url for BrowserContentHandler
+    const redirect = lazy.WindowsSetDefaultRedirect.consume(cmdArg);
+    if (!redirect) {
+      return;
+    }
+
+    const { overrideUri } = redirect;
+
+    lazy.logConsole.debug(
+      `Claimed IOpenWithLauncher openWithArg ${cmdArg}: state=${aCmdLine.state}, overrideUri=${overrideUri}`
+    );
+
+    // Consume the arg and suppress the default open so BrowserContentHandler
+    // doesn't act on it
+    aCmdLine.handleFlagWithParam("url", false);
+    aCmdLine.preventDefault = true;
+    if (overrideUri === null) {
+      return;
+    }
+
+    lazy.logConsole.info(
+      `Redirecting IOpenWithLauncher round-trip to ${overrideUri}`
+    );
+
+    try {
+      const win = lazy.BrowserWindowTracker.getTopWindow();
+      if (win) {
+        win.openTrustedLinkIn(overrideUri, "tab");
+        return;
+      }
+
+      const args = Cc["@mozilla.org/supports-string;1"].createInstance(
+        Ci.nsISupportsString
+      );
+      args.data = overrideUri;
+      lazy.BrowserWindowTracker.openWindow({ args });
+    } catch (e) {
+      lazy.logConsole.error(
+        `Failed to open redirect target ${overrideUri}:`,
+        e
+      );
+    }
+  }
+}
diff --git a/browser/components/shell/WindowsSetDefaultRedirect.sys.mjs b/browser/components/shell/WindowsSetDefaultRedirect.sys.mjs
new file mode 100644
index 000000000000..d2c8d5812ae7
--- /dev/null
+++ b/browser/components/shell/WindowsSetDefaultRedirect.sys.mjs
@@ -0,0 +1,135 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * The IOpenWithLauncher api call protocol shared by the producer
+ * (ShellService.setAsDefault{PDF,Protocol}Handler) and the consumer
+ * (WindowsSetDefaultAppCmdHandler).
+ *
+ * ShellService arms a one-shot redirect before launching the OS "Open with"
+ * picker via IOpenWithLauncher; once the user picks Firefox, the OS relaunches
+ * Firefox with the same value and the command-line handler consumes it. This
+ * module owns that shared state (the pref shape and the matching rules).
+ */
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  FileUtils: "resource://gre/modules/FileUtils.sys.mjs",
+});
+
+// This pref is an object { openWithArg, overrideUri, type } consumed by
+// WindowsSetDefaultAppCmdHandler when the user picks a default (file type or
+// protocol) using the IOpenWithLauncher API. It is reset anytime the dialog is
+// used again, or when we intercept the OS reopening one of our openWithArgs.
+export const SET_DEFAULT_REDIRECT_PREF =
+  "browser.shell.setDefaultApp.pendingRedirect";
+
+export class WindowsSetDefaultRedirect {
+  // Supported default types to set using IOpenWithLauncher.
+  static TYPE = {
+    FILE: 1 << 0,
+    PROTOCOL: 1 << 1,
+  };
+
+  /**
+   * Stash a one-shot redirect for the IOpenWithLauncher call.
+   *
+   * @param {string} openWithArg
+   *   The value handed to launchSetDefaultAppPicker, which the OS hands back as
+   *   "-osint -url <openWithArg>" once the user picks a new default. Depending on
+   *   type, this is either a file path on the system (file-type defaults) or a
+   *   URL (protocol defaults).
+   * @param {?string} overrideUri
+   *   URI spec to open when openWithArg comes back, or null to consume the relaunch
+   *   and open nothing.
+   * @param {number} type
+   *   One of WindowsSetDefaultRedirect.TYPE, identifying whether openWithArg is a
+   *   file path or a URL.
+   */
+  static arm(openWithArg, overrideUri, type) {
+    // Clear any stale object left by an older call.
+    Services.prefs.clearUserPref(SET_DEFAULT_REDIRECT_PREF);
+
+    Services.prefs.setStringPref(
+      SET_DEFAULT_REDIRECT_PREF,
+      JSON.stringify({ openWithArg, overrideUri: overrideUri ?? null, type })
+    );
+  }
+
+  /**
+   * Clear a pending redirect.
+   */
+  static clear() {
+    Services.prefs.clearUserPref(SET_DEFAULT_REDIRECT_PREF);
+  }
+
+  /**
+   * If `arg` is the openWithArg stashed by the most recent
+   * launchSetDefaultAppPicker call, consume the one-shot redirect and return
+   * its `{ overrideUri }`, where overrideUri is a URI spec to open or null to
+   * just suppress the relaunch. Returns null when `arg` is unrelated to a
+   * pending attempt to set a default.
+   *
+   * @param {string} arg - The -url value the OS handed back.
+   * @returns {?{overrideUri: ?string}}
+   */
+  static consume(arg) {
+    const state = this.#read();
+    if (!state || !this.#matches(state, arg)) {
+      return null;
+    }
+    Services.prefs.clearUserPref(SET_DEFAULT_REDIRECT_PREF);
+    return { overrideUri: state.overrideUri ?? null };
+  }
+
+  /**
+   * Read and validate the pending redirect stashed by arm().
+   *
+   * @returns {?{openWithArg: string, overrideUri: ?string, type: number}} The
+   * stored state, or null when the pref is unset, holds the wrong type, or is
+   * malformed JSON.
+   */
+  static #read() {
+    let raw;
+    try {
+      raw = Services.prefs.getStringPref(SET_DEFAULT_REDIRECT_PREF, "");
+    } catch (e) {
+      return null;
+    }
+    if (!raw) {
+      return null;
+    }
+    try {
+      const state = JSON.parse(raw);
+      return state && typeof state.openWithArg === "string" ? state : null;
+    } catch (e) {
+      return null;
+    }
+  }
+
+  /**
+   * Checks if the -url value the OS handed back matches the stashed redirect.
+   *
+   * @param {{openWithArg: string, type: number}} state - The stashed redirect.
+   * @param {string} arg - The -url value from the OS relaunch.
+   * @returns {boolean}
+   */
+  static #matches(state, arg) {
+    switch (state.type) {
+      case this.TYPE.PROTOCOL:
+        return state.openWithArg === arg;
+      case this.TYPE.FILE:
+        try {
+          return new lazy.FileUtils.File(state.openWithArg).equals(
+            new lazy.FileUtils.File(arg)
+          );
+        } catch (e) {
+          return false;
+        }
+      default:
+        return false;
+    }
+  }
+}
diff --git a/browser/components/shell/components.conf b/browser/components/shell/components.conf
new file mode 100644
index 000000000000..58a246cd8245
--- /dev/null
+++ b/browser/components/shell/components.conf
@@ -0,0 +1,18 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+Classes = []
+
+if buildconfig.substs["OS_ARCH"] == "WINNT":
+    Classes += [
+        {
+            "cid": "{da7de528-7a15-452e-b5a7-521099997ca1}",
+            "contract_ids": ["@mozilla.org/browser/windows-default-clh;1"],
+            "categories": {
+                "command-line-handler": "l-windows-default",
+            },
+            "esModule": "moz-src:///browser/components/shell/WindowsSetDefaultAppCmdHandler.sys.mjs",
+            "constructor": "CommandLineHandler",
+        },
+    ]
diff --git a/browser/components/shell/content/blank.pdf b/browser/components/shell/content/blank.pdf
new file mode 100644
index 000000000000..ffff6d4bcf19
Binary files /dev/null and b/browser/components/shell/content/blank.pdf differ
diff --git a/browser/components/shell/content/confused_fox.pdf b/browser/components/shell/content/confused_fox.pdf
new file mode 100644
index 000000000000..65b44d374489
Binary files /dev/null and b/browser/components/shell/content/confused_fox.pdf differ
diff --git a/browser/components/shell/moz.build b/browser/components/shell/moz.build
index 01fc43e1d938..55052eb44fe1 100644
--- a/browser/components/shell/moz.build
+++ b/browser/components/shell/moz.build
@@ -73,6 +73,15 @@ elif CONFIG["OS_ARCH"] == "WINNT":
         "crypt32",
         "propsys",
     ]
+    FINAL_TARGET_FILES += ["content/blank.pdf", "content/confused_fox.pdf"]
+
+    MOZ_SRC_FILES += [
+        "WindowsSetDefaultAppCmdHandler.sys.mjs",
+    ]
+
+    XPCOM_MANIFESTS += [
+        "components.conf",
+    ]
 
 XPIDL_MODULE = "shellservice"
 
@@ -83,6 +92,7 @@ MOZ_SRC_FILES += [
     "HeadlessShell.sys.mjs",
     "ScreenshotChild.sys.mjs",
     "ShellService.sys.mjs",
+    "WindowsSetDefaultRedirect.sys.mjs",
 ]
 
 MOZ_SRC_FILES += [
diff --git a/browser/components/shell/nsIWindowsShellService.idl b/browser/components/shell/nsIWindowsShellService.idl
index aae7ba134a1a..d34c6a1edabf 100644
--- a/browser/components/shell/nsIWindowsShellService.idl
+++ b/browser/components/shell/nsIWindowsShellService.idl
@@ -10,6 +10,27 @@ interface nsIFile;
 [scriptable, uuid(fb9b59db-5a91-4e67-92b6-35e7d6e6d3fd)]
 interface nsIWindowsShellService : nsIShellService
 {
+  /*
+   * Bit flags for launchSetDefaultAppPicker's aFlags argument. They map 1:1
+   * onto the flags accepted by the undocumented Windows IOpenWithLauncher COM
+   * interface's Launch method.
+   * The mapping below was determined empirically and determines the picker's
+   * messaging and actions.
+   */
+  // Suppresses the default open-in-selected-app behavior. Remote-controlled
+  // by Microsoft.
+  const long OPEN_WITH_SUPPRESS_OPEN = 1 << 2;
+  // Switches picker messaging to protocol mode ("Select a default handler
+  // for ___ links").
+  const long OPEN_WITH_PROTOCOL_MESSAGING = 1 << 3;
+  // Opens with the selected app just once; does not set a default.
+  const long OPEN_WITH_OPEN_ONCE = 1 << 6;
+  // Asks the user to set a default for the given file type or protocol. Messaging is
+  // ideal on Windows 11 only.
+  const long OPEN_WITH_SET_HANDLER = 1 << 7;
+  // Like OPEN_WITH_SET_HANDLER, but with ideal messaging for Windows 10.
+  const long OPEN_WITH_SET_HANDLER_WIN10 = 1 << 13;
+
   /*
    * Creates a new shortcut (.lnk) file. This shortcut will be recorded in
    * a new shortcuts log file located in %PROGRAMDATA%\Mozilla-1de4eec8-1241-4177-a864-e594e8d1fb38
@@ -349,16 +370,19 @@ interface nsIWindowsShellService : nsIShellService
   );
 
   /*
-   * Launch the default app picker for a given file type via the Windows
-   * IOpenWithLauncher COM interface.
+   * Launch the default app picker via the Windows IOpenWithLauncher COM
+   * interface. The caller picks the target (a file path whose extension
+   * determines the picker for file-type defaults, or a URL for protocol
+   * defaults) and the flag bits that drive the picker's messaging and actions.
    *
-   * @param aFileType
-   *        The file extension (e.g. ".pdf") for which to open
-   *        the default app picker.
+   * @param aTarget
+   *        File path or URL.
+   * @param aFlags
+   *        Flag combination that determines the behavior of the IOpenWithLauncher dialog.
    * @throws NS_ERROR_FAILURE
    *         If IOpenWithLauncher is unavailable or fails.
    */
-  void launchOpenWithDefaultPickerForFileType(in AString aFileType);
+  void launchSetDefaultAppPicker(in AString aTarget, in long aFlags);
 
   /*
    * Open the Windows modern settings dialog for choosing default apps
diff --git a/browser/components/shell/nsWindowsShellService.cpp b/browser/components/shell/nsWindowsShellService.cpp
index b2b1bc836114..2b36517e5bd9 100644
--- a/browser/components/shell/nsWindowsShellService.cpp
+++ b/browser/components/shell/nsWindowsShellService.cpp
@@ -459,13 +459,21 @@ nsWindowsShellService::CanSetDefaultBrowserUserChoice(bool* aResult) {
 
 class __declspec(novtable) IOpenWithLauncher : public IUnknown {
  public:
+  // lpszPath selects what the picker offers to set as default. It accepts
+  // several shapes:
+  //   - a file path:     "C:\\path\\to\\file.pdf"
+  //   - a file type:     ".pdf"
+  //   - a protocol:      "http"
+  //   - a protocol URI:  "https://example.com", "mailto:foo@example.com"
+  // flags determines the messaging and actions available of the
+  // IOpenWithLauncher dialog.
   virtual HRESULT STDMETHODCALLTYPE Launch(HWND hWndParent, LPCWSTR lpszPath,
                                            int flags) = 0;
 };
 
 NS_IMETHODIMP
-nsWindowsShellService::LaunchOpenWithDefaultPickerForFileType(
-    const nsAString& aFileType) {
+nsWindowsShellService::LaunchSetDefaultAppPicker(const nsAString& aTarget,
+                                                 int32_t aFlags) {
   static constexpr GUID IID_IOpenWithLauncher = {
       0x6a283fe2,
       0xecfa,
@@ -500,10 +508,7 @@ nsWindowsShellService::LaunchOpenWithDefaultPickerForFileType(
   // Make sure the dialog is foregrounded.
   CoAllowSetForegroundWindow(pOWL, nullptr);
 
-  // The flag is a bit of a mystery; on Win11+ 0x84 gives ideal messaging, on
-  // Win10 we use 0x2004.
-  int flag = mozilla::IsWin11OrLater() ? 0x84 : 0x2004;
-  hr = pOWL->Launch(nullptr, aFileType.Data(), flag);
+  hr = pOWL->Launch(nullptr, PromiseFlatString(aTarget).get(), aFlags);
 
   return SUCCEEDED(hr) ? NS_OK : NS_ERROR_FAILURE;
 }
diff --git a/browser/components/shell/test/browser.toml b/browser/components/shell/test/browser.toml
index 11e34f2bba1c..c409640d839e 100644
--- a/browser/components/shell/test/browser.toml
+++ b/browser/components/shell/test/browser.toml
@@ -120,3 +120,9 @@ tags = "os_integration"
 ["browser_setDesktopBackgroundPreview.js"]
 support-files = ["large.png", "canvas.html"]
 tags = "os_integration"
+
+["browser_windowsSetDefaultAppCmdHandler.js"]
+run-if = [
+  "os == 'win'",
+]
+tags = "os_integration"
diff --git a/browser/components/shell/test/browser_setDefaultPDFHandler.js b/browser/components/shell/test/browser_setDefaultPDFHandler.js
index 04cc1f069303..3ffa6cb573c4 100644
--- a/browser/components/shell/test/browser_setDefaultPDFHandler.js
+++ b/browser/components/shell/test/browser_setDefaultPDFHandler.js
@@ -8,6 +8,9 @@ ChromeUtils.defineESModuleGetters(this, {
   sinon: "resource://testing-common/Sinon.sys.mjs",
 });
 
+const confusedFoxPath = ShellService.getBundledPdfFile("confused_fox.pdf").path;
+const SET_HANDLER_WIN11 = Ci.nsIWindowsShellService.OPEN_WITH_SET_HANDLER;
+
 const setDefaultBrowserUserChoiceStub = sinon.stub();
 const setDefaultExtensionHandlersUserChoiceStub = sinon
   .stub()
@@ -34,17 +37,15 @@ const _userChoiceImpossibleTelemetryResultStub = sinon
 const setDefaultStub = sinon.stub();
 // We'll dynamically update this as needed during the tests.
 const queryCurrentDefaultHandlerForStub = sinon.stub();
-const launchOpenWithDefaultPickerForFileTypeStub = sinon.stub();
+const launchSetDefaultAppPickerStub = sinon.stub();
 const launchModernSettingsDialogDefaultAppsStub = sinon.stub();
 const shellStub = sinon.stub(ShellService, "shellService").value({
   setDefaultBrowser: setDefaultStub,
   queryCurrentDefaultHandlerFor: queryCurrentDefaultHandlerForStub,
-  QueryInterface: () => ({
-    launchOpenWithDefaultPickerForFileType:
-      launchOpenWithDefaultPickerForFileTypeStub,
-    launchModernSettingsDialogDefaultApps:
-      launchModernSettingsDialogDefaultAppsStub,
-  }),
+  launchSetDefaultAppPicker: launchSetDefaultAppPickerStub,
+  launchModernSettingsDialogDefaultApps:
+    launchModernSettingsDialogDefaultAppsStub,
+  QueryInterface: ChromeUtils.generateQI([]),
 });
 
 registerCleanupFunction(() => {
@@ -228,7 +229,7 @@ add_task(async function test_setAsDefaultPDFHandler_knownBrowser() {
   const expectedArguments = [aumi, [".pdf", "FirefoxPDF"]];
   const resetStubs = () => {
     setDefaultExtensionHandlersUserChoiceStub.resetHistory();
-    launchOpenWithDefaultPickerForFileTypeStub.resetHistory();
+    launchSetDefaultAppPickerStub.resetHistory();
     launchModernSettingsDialogDefaultAppsStub.resetHistory();
   };
 
@@ -250,7 +251,7 @@ add_task(async function test_setAsDefaultPDFHandler_knownBrowser() {
       "Called default browser agent with expected arguments"
     );
     Assert.ok(
-      launchOpenWithDefaultPickerForFileTypeStub.notCalled,
+      launchSetDefaultAppPickerStub.notCalled,
       "Did not fall back to open-with picker"
     );
     Assert.ok(
@@ -271,7 +272,7 @@ add_task(async function test_setAsDefaultPDFHandler_knownBrowser() {
       "Called default browser agent with expected arguments"
     );
     Assert.ok(
-      launchOpenWithDefaultPickerForFileTypeStub.notCalled,
+      launchSetDefaultAppPickerStub.notCalled,
       "Did not fall back to open-with picker"
     );
     Assert.ok(
@@ -289,7 +290,7 @@ add_task(async function test_setAsDefaultPDFHandler_knownBrowser() {
       "Did not use userChoice"
     );
     Assert.ok(
-      launchOpenWithDefaultPickerForFileTypeStub.notCalled,
+      launchSetDefaultAppPickerStub.notCalled,
       "Did not fall back to open-with picker"
     );
     Assert.ok(
@@ -310,7 +311,7 @@ add_task(async function test_setAsDefaultPDFHandler_knownBrowser() {
       "Called default browser agent with expected arguments"
     );
     Assert.ok(
-      launchOpenWithDefaultPickerForFileTypeStub.notCalled,
+      launchSetDefaultAppPickerStub.notCalled,
       "Did not fall back to open-with picker"
     );
     Assert.ok(
@@ -365,8 +366,11 @@ add_task(async function test_setAsDefaultPDFHandler_fallback() {
 
     Assert.ok(userChoiceStub.called, "Attempted userChoice");
     Assert.ok(
-      launchOpenWithDefaultPickerForFileTypeStub.calledWith(".pdf"),
-      "Fell back to open-with picker for .pdf"
+      launchSetDefaultAppPickerStub.calledWith(
+        confusedFoxPath,
+        SET_HANDLER_WIN11
+      ),
+      "Fell back to open-with picker with bundled PDF path and Win11 flag"
     );
     Assert.ok(
       launchModernSettingsDialogDefaultAppsStub.notCalled,
@@ -392,7 +396,7 @@ add_task(async function test_setAsDefaultPDFHandler_fallback() {
     );
     userChoiceStub.resetHistory();
     isDefaultHandlerForStub.resetHistory();
-    launchOpenWithDefaultPickerForFileTypeStub.resetHistory();
+    launchSetDefaultAppPickerStub.resetHistory();
     launchModernSettingsDialogDefaultAppsStub.resetHistory();
 
     info(
@@ -413,22 +417,25 @@ add_task(async function test_setAsDefaultPDFHandler_fallback() {
     isDefaultHandlerForStub.returns(true);
     userChoiceStub.resetHistory();
     isDefaultHandlerForStub.resetHistory();
-    launchOpenWithDefaultPickerForFileTypeStub.resetHistory();
+    launchSetDefaultAppPickerStub.resetHistory();
     launchModernSettingsDialogDefaultAppsStub.resetHistory();
 
     info(
       "When userChoice fails and open-with picker fails, should fall back to settings dialog"
     );
     Services.fog.testResetFOG();
-    launchOpenWithDefaultPickerForFileTypeStub.throws(
+    launchSetDefaultAppPickerStub.throws(
       new Error("mock IOpenWithLauncher failure")
     );
     await ShellService.setAsDefaultPDFHandler(false);
 
     Assert.ok(userChoiceStub.called, "Attempted userChoice");
     Assert.ok(
-      launchOpenWithDefaultPickerForFileTypeStub.calledWith(".pdf"),
-      "Attempted open-with picker for .pdf"
+      launchSetDefaultAppPickerStub.calledWith(
+        confusedFoxPath,
+        SET_HANDLER_WIN11
+      ),
+      "Attempted open-with picker with bundled PDF path and Win11 flag"
     );
     Assert.ok(
       launchModernSettingsDialogDefaultAppsStub.called,
@@ -463,7 +470,7 @@ add_task(async function test_setAsDefaultPDFHandler_fallback() {
     );
     userChoiceStub.resetHistory();
     isDefaultHandlerForStub.resetHistory();
-    launchOpenWithDefaultPickerForFileTypeStub.resetHistory();
+    launchSetDefaultAppPickerStub.resetHistory();
     launchModernSettingsDialogDefaultAppsStub.resetHistory();
 
     info(
@@ -499,7 +506,7 @@ add_task(async function test_setAsDefaultPDFHandler_fallback() {
       "Event result_is_default is false when no method set the default"
     );
   } finally {
-    launchOpenWithDefaultPickerForFileTypeStub.reset();
+    launchSetDefaultAppPickerStub.reset();
     launchModernSettingsDialogDefaultAppsStub.reset();
     sandbox.restore();
     await SpecialPowers.popPrefEnv();
@@ -529,7 +536,7 @@ add_task(async function test_setAsDefaultPDFHandler_useOpenWithDisabled() {
     await ShellService.setAsDefaultPDFHandler(false);
 
     Assert.ok(
-      launchOpenWithDefaultPickerForFileTypeStub.notCalled,
+      launchSetDefaultAppPickerStub.notCalled,
       "Did not invoke open-with picker when pref is disabled"
     );
     Assert.ok(
@@ -550,7 +557,7 @@ add_task(async function test_setAsDefaultPDFHandler_useOpenWithDisabled() {
       "Event result_is_default reflects isDefaultHandlerFor"
     );
   } finally {
-    launchOpenWithDefaultPickerForFileTypeStub.reset();
+    launchSetDefaultAppPickerStub.reset();
     launchModernSettingsDialogDefaultAppsStub.reset();
     sandbox.restore();
     await SpecialPowers.popPrefEnv();
diff --git a/browser/components/shell/test/browser_windowsSetDefaultAppCmdHandler.js b/browser/components/shell/test/browser_windowsSetDefaultAppCmdHandler.js
new file mode 100644
index 000000000000..62c31979f9c3
--- /dev/null
+++ b/browser/components/shell/test/browser_windowsSetDefaultAppCmdHandler.js
@@ -0,0 +1,293 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { CommandLineHandler } = ChromeUtils.importESModule(
+  "moz-src:///browser/components/shell/WindowsSetDefaultAppCmdHandler.sys.mjs"
+);
+
+ChromeUtils.defineESModuleGetters(this, {
+  BrowserWindowTracker: "resource:///modules/BrowserWindowTracker.sys.mjs",
+  SET_DEFAULT_REDIRECT_PREF:
+    "moz-src:///browser/components/shell/WindowsSetDefaultRedirect.sys.mjs",
+  ShellService: "moz-src:///browser/components/shell/ShellService.sys.mjs",
+  WindowsSetDefaultRedirect:
+    "moz-src:///browser/components/shell/WindowsSetDefaultRedirect.sys.mjs",
+  sinon: "resource://testing-common/Sinon.sys.mjs",
+});
+
+Assert.equal(AppConstants.platform, "win", "Platform is Windows");
+
+const confusedFoxPath = ShellService.getBundledPdfFile("confused_fox.pdf").path;
+const blankURISpec = Services.io.newFileURI(
+  ShellService.getBundledPdfFile("blank.pdf")
+).spec;
+
+const workingDir = Services.dirsvc.get("GreD", Ci.nsIFile);
+
+// Build a real nsICommandLine from an args array, so we exercise the same
+// flag-parsing the OS-initiated launch goes through
+function makeCmdLine(args, state = Ci.nsICommandLine.STATE_INITIAL_LAUNCH) {
+  return Cu.createCommandLine(args, workingDir, state);
+}
+
+// Arm the one-shot redirect via the real ShellService helper, the way
+// setAsDefault{*}Handler does right before launching the OS picker. These
+// tests all use file openWithArgs, so default the type.
+function armRedirect(
+  openWithArg,
+  overrideUri,
+  type = WindowsSetDefaultRedirect.TYPE.FILE
+) {
+  WindowsSetDefaultRedirect.arm(openWithArg, overrideUri, type);
+}
+
+let fakeWin;
+let getTopWindowStub;
+let openWindowStub;
+
+add_setup(function () {
+  fakeWin = { openTrustedLinkIn: sinon.stub() };
+  // Stubbed rather than spied: getTopWindow must return our fake window (and
+  // null on demand), and openWindow must not actually open a window in the
+  // test harness. We only inspect the calls.
+  getTopWindowStub = sinon.stub(BrowserWindowTracker, "getTopWindow");
+  openWindowStub = sinon.stub(BrowserWindowTracker, "openWindow");
+});
+
+registerCleanupFunction(() => {
+  sinon.restore();
+  Services.prefs.clearUserPref(SET_DEFAULT_REDIRECT_PREF);
+});
+
+function resetState() {
+  fakeWin.openTrustedLinkIn.resetHistory();
+  getTopWindowStub.reset();
+  getTopWindowStub.returns(fakeWin);
+  openWindowStub.reset();
+  Services.prefs.clearUserPref(SET_DEFAULT_REDIRECT_PREF);
+}
+
+add_task(async function test_no_osint_returns_early() {
+  resetState();
+  armRedirect(confusedFoxPath, blankURISpec);
+  const cmdLine = makeCmdLine(["-url", confusedFoxPath]);
+  new CommandLineHandler().handle(cmdLine);
+
+  Assert.equal(
+    cmdLine.preventDefault,
+    false,
+    "preventDefault left untouched without -osint"
+  );
+  Assert.greaterOrEqual(
+    cmdLine.findFlag("url", false),
+    0,
+    "-url remains for the next handler"
+  );
+  Assert.ok(
+    fakeWin.openTrustedLinkIn.notCalled,
+    "No tab opened without -osint"
+  );
+  Assert.ok(openWindowStub.notCalled, "No window opened without -osint");
+});
+
+add_task(async function test_osint_without_url_returns_early() {
+  resetState();
+  armRedirect(confusedFoxPath, blankURISpec);
+  const cmdLine = makeCmdLine(["-osint"]);
+  new CommandLineHandler().handle(cmdLine);
+
+  Assert.equal(
+    cmdLine.preventDefault,
+    false,
+    "preventDefault left untouched without -url"
+  );
+  Assert.ok(fakeWin.openTrustedLinkIn.notCalled, "No tab opened without -url");
+  Assert.ok(openWindowStub.notCalled, "No window opened without -url");
+});
+
+add_task(async function test_no_pending_redirect_leaves_arg() {
+  // No armed redirect: even a -url that looks like our stub PDF is a real,
+  // user-initiated open (e.g. they double-clicked the file), so we must leave
+  // it for BrowserContentHandler.
+  resetState();
+
+  const cmdLine = makeCmdLine(["-osint", "-url", confusedFoxPath]);
+  new CommandLineHandler().handle(cmdLine);
+
+  Assert.equal(
+    cmdLine.preventDefault,
+    false,
+    "preventDefault not set when no redirect is pending"
+  );
+  Assert.greaterOrEqual(
+    cmdLine.findFlag("url", false),
+    0,
+    "-url preserved for BrowserContentHandler"
+  );
+  Assert.ok(
+    fakeWin.openTrustedLinkIn.notCalled,
+    "No tab opened when no redirect is pending"
+  );
+  Assert.ok(
+    openWindowStub.notCalled,
+    "No window opened when no redirect is pending"
+  );
+});
+
+add_task(async function test_unrelated_url_arg_is_ignored() {
+  resetState();
+  armRedirect(confusedFoxPath, blankURISpec);
+
+  const cmdLine = makeCmdLine([
+    "-osint",
+    "-url",
+    "https://example.com/some-page",
+  ]);
+  new CommandLineHandler().handle(cmdLine);
+
+  Assert.equal(
+    cmdLine.preventDefault,
+    false,
+    "preventDefault not set for a -url that isn't the pending openWithArg"
+  );
+  Assert.greaterOrEqual(
+    cmdLine.findFlag("url", false),
+    0,
+    "-url preserved for subsequent handler"
+  );
+  Assert.ok(
+    fakeWin.openTrustedLinkIn.notCalled,
+    "No tab opened for an unrelated -url"
+  );
+  Assert.ok(
+    Services.prefs.prefHasUserValue(SET_DEFAULT_REDIRECT_PREF),
+    "Pending redirect untouched when the openWithArg doesn't match"
+  );
+});
+
+add_task(async function test_suppress_only_when_target_null() {
+  resetState();
+  armRedirect(confusedFoxPath, null);
+
+  const cmdLine = makeCmdLine(["-osint", "-url", confusedFoxPath]);
+  new CommandLineHandler().handle(cmdLine);
+
+  Assert.equal(
+    cmdLine.preventDefault,
+    true,
+    "openWithArg suppressed so BrowserContentHandler skips it"
+  );
+  Assert.equal(
+    cmdLine.findFlag("url", false),
+    -1,
+    "-url consumed even with no redirect target"
+  );
+  Assert.ok(
+    fakeWin.openTrustedLinkIn.notCalled,
+    "No redirect when target is null"
+  );
+  Assert.ok(openWindowStub.notCalled, "No fallback window when target is null");
+  Assert.ok(
+    !Services.prefs.prefHasUserValue(SET_DEFAULT_REDIRECT_PREF),
+    "Redirect intent consumed"
+  );
+});
+
+add_task(async function test_redirects_to_top_window() {
+  resetState();
+  armRedirect(confusedFoxPath, blankURISpec);
+
+  const cmdLine = makeCmdLine(["-osint", "-url", confusedFoxPath]);
+  new CommandLineHandler().handle(cmdLine);
+
+  Assert.equal(
+    cmdLine.preventDefault,
+    true,
+    "Default open suppressed for the pending openWithArg"
+  );
+  Assert.equal(cmdLine.findFlag("url", false), -1, "-url consumed");
+  Assert.ok(
+    fakeWin.openTrustedLinkIn.calledOnce,
+    "Redirected into the top window"
+  );
+  Assert.deepEqual(fakeWin.openTrustedLinkIn.firstCall.args, [
+    blankURISpec,
+    "tab",
+  ]);
+  Assert.ok(openWindowStub.notCalled, "No new window when one exists");
+  Assert.ok(
+    !Services.prefs.prefHasUserValue(SET_DEFAULT_REDIRECT_PREF),
+    "Redirect intent is one-shot and cleared after use"
+  );
+});
+
+add_task(async function test_opens_new_window_when_no_top() {
+  resetState();
+  getTopWindowStub.returns(null);
+  armRedirect(confusedFoxPath, blankURISpec);
+
+  const cmdLine = makeCmdLine(["-osint", "-url", confusedFoxPath]);
+  new CommandLineHandler().handle(cmdLine);
+
+  Assert.equal(
+    cmdLine.preventDefault,
+    true,
+    "Default open suppressed even when no top window exists"
+  );
+  Assert.ok(
+    fakeWin.openTrustedLinkIn.notCalled,
+    "Top-window path skipped when getTopWindow returns null"
+  );
+  Assert.ok(openWindowStub.calledOnce, "Falls back to openWindow");
+
+  const opts = openWindowStub.firstCall.args[0];
+  Assert.ok(
+    opts && opts.args,
+    "openWindow called with a {args} options object"
+  );
+  Assert.ok(
+    opts.args instanceof Ci.nsISupportsString,
+    "args is an nsISupportsString"
+  );
+  Assert.equal(
+    opts.args.data,
+    blankURISpec,
+    "nsISupportsString carries the redirect URI"
+  );
+  Assert.ok(
+    !Services.prefs.prefHasUserValue(SET_DEFAULT_REDIRECT_PREF),
+    "Redirect intent is one-shot and cleared after use"
+  );
+});
+
+add_task(async function test_intent_is_one_shot() {
+  resetState();
+  armRedirect(confusedFoxPath, blankURISpec);
+
+  new CommandLineHandler().handle(
+    makeCmdLine(["-osint", "-url", confusedFoxPath])
+  );
+  Assert.ok(
+    fakeWin.openTrustedLinkIn.calledOnce,
+    "First call honors the pending redirect"
+  );
+
+  fakeWin.openTrustedLinkIn.resetHistory();
+  const second = makeCmdLine(["-osint", "-url", confusedFoxPath]);
+  new CommandLineHandler().handle(second);
+
+  Assert.equal(
+    second.preventDefault,
+    false,
+    "Second call leaves the openWithArg alone because the intent was consumed"
+  );
+  Assert.greaterOrEqual(
+    second.findFlag("url", false),
+    0,
+    "-url preserved on the second call (no pending redirect)"
+  );
+  Assert.ok(
+    fakeWin.openTrustedLinkIn.notCalled,
+    "Second call does not redirect"
+  );
+});
diff --git a/browser/installer/package-manifest.in b/browser/installer/package-manifest.in
index 83e52b260c38..203811a0fb98 100644
--- a/browser/installer/package-manifest.in
+++ b/browser/installer/package-manifest.in
@@ -221,6 +221,13 @@
 @BINPATH@/@MOZ_DXC_DLL_NAME@
 #endif
 
+; Bundled stub PDFs handed to the OS "Open with" picker by the
+; set-default-PDF-handler flow (see WindowsSetDefaultAppCmdHandler).
+#ifdef XP_WIN
+@RESPATH@/browser/confused_fox.pdf
+@RESPATH@/browser/blank.pdf
+#endif
+
 ; [Browser Chrome Files]
 @RESPATH@/browser/chrome.manifest
 @RESPATH@/browser/chrome/browser@JAREXT@
diff --git a/toolkit/components/messaging-system/lib/SpecialMessageActions.sys.mjs b/toolkit/components/messaging-system/lib/SpecialMessageActions.sys.mjs
index 6aca21afcc4a..ad02aa48f3b8 100644
--- a/toolkit/components/messaging-system/lib/SpecialMessageActions.sys.mjs
+++ b/toolkit/components/messaging-system/lib/SpecialMessageActions.sys.mjs
@@ -185,8 +185,14 @@ export const SpecialMessageActions = {
    *
    * @param {Window} window Reference to a window object
    */
-  async setDefaultPDFHandler(window, onlyIfKnownBrowser = false) {
-    await window.getShellService().setAsDefaultPDFHandler(onlyIfKnownBrowser);
+  async setDefaultPDFHandler(
+    window,
+    onlyIfKnownBrowser = false,
+    openInFirefox = false
+  ) {
+    await window
+      .getShellService()
+      .setAsDefaultPDFHandler(onlyIfKnownBrowser, openInFirefox);
   },
 
   /**
@@ -815,7 +821,8 @@ export const SpecialMessageActions = {
       case "SET_DEFAULT_PDF_HANDLER":
         await this.setDefaultPDFHandler(
           window,
-          action.data?.onlyIfKnownBrowser ?? false
+          action.data?.onlyIfKnownBrowser ?? false,
+          action.data?.openInFirefox ?? false
         );
         break;
       case "DECLINE_DEFAULT_PDF_HANDLER":
diff --git a/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/SpecialMessageActionSchemas.json b/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/SpecialMessageActionSchemas.json
index beb347d42f7e..01270166f1ab 100644
--- a/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/SpecialMessageActionSchemas.json
+++ b/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/SpecialMessageActionSchemas.json
@@ -478,6 +478,10 @@
                 "onlyIfKnownBrowser": {
                   "type": "boolean",
                   "description": "Only set Firefox as the default PDF handler if the current PDF handler is a known browser."
+                },
+                "openInFirefox": {
+                  "type": "boolean",
+                  "description": "If the OS hands the stub PDF back to Firefox after the user picks Firefox in the open-with dialog, open a follow-up PDF in a new tab instead of suppressing the launch."
                 }
               },
               "additionalProperties": false
diff --git a/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/index.md b/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/index.md
index c3daa6fa36e7..49322eeb6816 100644
--- a/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/index.md
+++ b/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/index.md
@@ -267,6 +267,10 @@ Windows only.
   // Only set Firefox as the default PDF handler if the current PDF handler is a
   // known browser.
   onlyIfKnownBrowser?: boolean;
+  // If the OS hands the stub PDF back to Firefox after the user picks Firefox
+  // in the open-with dialog, open a follow-up PDF in a new tab instead of
+  // suppressing the launch.
+  openInFirefox?: boolean;
 }
 ```
 
diff --git a/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/test/browser/browser_sma_default_pdf_handler.js b/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/test/browser/browser_sma_default_pdf_handler.js
index edb26cc3c296..012e6b9d0295 100644
--- a/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/test/browser/browser_sma_default_pdf_handler.js
+++ b/toolkit/components/messaging-system/schemas/SpecialMessageActionSchemas/test/browser/browser_sma_default_pdf_handler.js
@@ -24,7 +24,7 @@ add_task(async function test_set_default_pdf_handler_no_data() {
     "setAsDefaultPDFHandler was called by the action"
   );
   Assert.ok(
-    stub.calledWithExactly(false),
+    stub.calledWithExactly(false, false),
     "setAsDefaultPDFHandler called with onlyIfKnownBrowser = false"
   );
 });
@@ -55,7 +55,7 @@ add_task(async function test_set_default_pdf_handler_data_false() {
     "setAsDefaultPDFHandler was called by the action"
   );
   Assert.ok(
-    stub.calledWithExactly(false),
+    stub.calledWithExactly(false, false),
     "setAsDefaultPDFHandler called with onlyIfKnownBrowser = false"
   );
 });
@@ -86,7 +86,7 @@ add_task(async function test_set_default_pdf_handler_data_true() {
     "setAsDefaultPDFHandler was called by the action"
   );
   Assert.ok(
-    stub.calledWithExactly(true),
+    stub.calledWithExactly(true, false),
     "setAsDefaultPDFHandler called with onlyIfKnownBrowser = true"
   );
 });