diff --git a/devtools/client/netmonitor/src/utils/request-utils.js b/devtools/client/netmonitor/src/utils/request-utils.js
index 564fdda28abe..7e5d45d56497 100644
--- a/devtools/client/netmonitor/src/utils/request-utils.js
+++ b/devtools/client/netmonitor/src/utils/request-utils.js
@@ -93,8 +93,8 @@ async function getFormDataSections(
  * @return {object} a headers object with updated content payload
  */
 async function fetchHeaders(headers, getLongString) {
-  for (const { value } of headers.headers) {
-    headers.headers.value = await getLongString(value);
+  for (const header of headers.headers) {
+    header.value = await getLongString(header.value);
   }
   return headers;
 }
diff --git a/devtools/client/netmonitor/test/xpcshell/test_request-utils-fetchHeaders.js b/devtools/client/netmonitor/test/xpcshell/test_request-utils-fetchHeaders.js
new file mode 100644
index 000000000000..e34889c2498a
--- /dev/null
+++ b/devtools/client/netmonitor/test/xpcshell/test_request-utils-fetchHeaders.js
@@ -0,0 +1,54 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { require } = ChromeUtils.importESModule(
+  "resource://devtools/shared/loader/Loader.sys.mjs"
+);
+const {
+  fetchHeaders,
+} = require("resource://devtools/client/netmonitor/src/utils/request-utils.js");
+
+add_task(async function test_fetchHeaders_resolves_long_string_values() {
+  const longCookieValue = "a=1; ".repeat(2001);
+
+  const mockLongStringGrip = {
+    type: "longString",
+    initial: longCookieValue.substring(0, 1000),
+    length: longCookieValue.length,
+  };
+
+  const headers = {
+    headers: [
+      { name: "User-Agent", value: "Mozilla/5.0" },
+      { name: "Cookie", value: mockLongStringGrip },
+    ],
+    headersSize: 12345,
+  };
+
+  async function mockGetLongString(value) {
+    if (typeof value === "object" && value.type === "longString") {
+      return longCookieValue;
+    }
+    return value;
+  }
+
+  const result = await fetchHeaders(headers, mockGetLongString);
+
+  equal(
+    result.headers[0].value,
+    "Mozilla/5.0",
+    "Short string header value is preserved"
+  );
+  equal(
+    typeof result.headers[1].value,
+    "string",
+    "Long string Cookie header value is resolved to a string"
+  );
+  equal(
+    result.headers[1].value,
+    longCookieValue,
+    "Long string Cookie header value is correctly resolved to its full content"
+  );
+});
diff --git a/devtools/client/netmonitor/test/xpcshell/xpcshell.toml b/devtools/client/netmonitor/test/xpcshell/xpcshell.toml
index 087f3d10a162..6630349a5611 100644
--- a/devtools/client/netmonitor/test/xpcshell/xpcshell.toml
+++ b/devtools/client/netmonitor/test/xpcshell/xpcshell.toml
@@ -8,6 +8,8 @@ run-if = [
 
 ["test_doc-utils.js"]
 
+["test_request-utils-fetchHeaders.js"]
+
 ["test_request-utils-fetchNetworkUpdatePacket.js"]
 
 ["test_request-utils-js-getFormattedProtocol.js"]