diff --git a/dom/filesystem/PFileSystemRequest.ipdl b/dom/filesystem/PFileSystemRequest.ipdl
index 7ed4891cc4fb..43dc7d420f26 100644
--- a/dom/filesystem/PFileSystemRequest.ipdl
+++ b/dom/filesystem/PFileSystemRequest.ipdl
@@ -62,7 +62,7 @@ union FileSystemResponseValue
   FileSystemErrorResponse;
 };
 
-[ChildImpl="FileSystemTaskChildBase"]
+[ChildImpl="mozilla::dom::FileSystemTaskChildBase"]
 protocol PFileSystemRequest
 {
   manager PBackground;
diff --git a/dom/indexedDB/PBackgroundIDBCursor.ipdl b/dom/indexedDB/PBackgroundIDBCursor.ipdl
index e7669f2554f5..66c73a72ec07 100644
--- a/dom/indexedDB/PBackgroundIDBCursor.ipdl
+++ b/dom/indexedDB/PBackgroundIDBCursor.ipdl
@@ -79,7 +79,7 @@ union CursorResponse
   IndexKeyCursorResponse[];
 };
 
-[ChildImpl="indexedDB::BackgroundCursorChildBase", ParentImpl=virtual]
+[ChildImpl="mozilla::dom::indexedDB::BackgroundCursorChildBase", ParentImpl=virtual]
 protocol PBackgroundIDBCursor
 {
   manager PBackgroundIDBTransaction or PBackgroundIDBVersionChangeTransaction;
diff --git a/dom/indexedDB/PBackgroundIDBDatabase.ipdl b/dom/indexedDB/PBackgroundIDBDatabase.ipdl
index 88d5e793926b..2514bfc506b4 100644
--- a/dom/indexedDB/PBackgroundIDBDatabase.ipdl
+++ b/dom/indexedDB/PBackgroundIDBDatabase.ipdl
@@ -26,7 +26,7 @@ namespace mozilla {
 namespace dom {
 namespace indexedDB {
 
-[ChildImpl="indexedDB::BackgroundDatabaseChild", ParentImpl=virtual]
+[ChildImpl="mozilla::dom::indexedDB::BackgroundDatabaseChild", ParentImpl=virtual]
 sync protocol PBackgroundIDBDatabase
 {
   manager PBackgroundIDBFactory;
diff --git a/dom/indexedDB/PBackgroundIDBFactory.ipdl b/dom/indexedDB/PBackgroundIDBFactory.ipdl
index c838189b2ba2..508832662cfe 100644
--- a/dom/indexedDB/PBackgroundIDBFactory.ipdl
+++ b/dom/indexedDB/PBackgroundIDBFactory.ipdl
@@ -46,7 +46,7 @@ union GetDatabasesResponse
   DatabaseMetadata[];
 };
 
-[ChildImpl="indexedDB::BackgroundFactoryChild", ParentImpl=virtual]
+[ChildImpl="mozilla::dom::indexedDB::BackgroundFactoryChild", ParentImpl=virtual]
 sync protocol PBackgroundIDBFactory
 {
   manager PBackground;
diff --git a/dom/indexedDB/PBackgroundIDBFactoryRequest.ipdl b/dom/indexedDB/PBackgroundIDBFactoryRequest.ipdl
index d85e7741b514..b8c644389340 100644
--- a/dom/indexedDB/PBackgroundIDBFactoryRequest.ipdl
+++ b/dom/indexedDB/PBackgroundIDBFactoryRequest.ipdl
@@ -30,7 +30,7 @@ union FactoryRequestResponse
   DeleteDatabaseRequestResponse;
 };
 
-[ManualDealloc, ChildImpl="indexedDB::BackgroundFactoryRequestChild", ParentImpl=virtual]
+[ManualDealloc, ChildImpl="mozilla::dom::indexedDB::BackgroundFactoryRequestChild", ParentImpl=virtual]
 protocol PBackgroundIDBFactoryRequest
 {
   manager PBackgroundIDBFactory;
diff --git a/dom/indexedDB/PBackgroundIDBRequest.ipdl b/dom/indexedDB/PBackgroundIDBRequest.ipdl
index 9b151316551c..09ff0b40ff75 100644
--- a/dom/indexedDB/PBackgroundIDBRequest.ipdl
+++ b/dom/indexedDB/PBackgroundIDBRequest.ipdl
@@ -160,7 +160,7 @@ union PreprocessResponse
   ObjectStoreGetAllPreprocessResponse;
 };
 
-[ManualDealloc, ChildImpl="indexedDB::BackgroundRequestChild", ParentImpl=virtual]
+[ManualDealloc, ChildImpl="mozilla::dom::indexedDB::BackgroundRequestChild", ParentImpl=virtual]
 protocol PBackgroundIDBRequest
 {
   manager PBackgroundIDBTransaction or PBackgroundIDBVersionChangeTransaction;
diff --git a/dom/indexedDB/PBackgroundIDBTransaction.ipdl b/dom/indexedDB/PBackgroundIDBTransaction.ipdl
index 071e031cb238..7d9447be6c4a 100644
--- a/dom/indexedDB/PBackgroundIDBTransaction.ipdl
+++ b/dom/indexedDB/PBackgroundIDBTransaction.ipdl
@@ -15,7 +15,7 @@ namespace mozilla {
 namespace dom {
 namespace indexedDB {
 
-[ChildImpl="indexedDB::BackgroundTransactionChild", ParentImpl=virtual]
+[ChildImpl="mozilla::dom::indexedDB::BackgroundTransactionChild", ParentImpl=virtual]
 protocol PBackgroundIDBTransaction {
   manager PBackgroundIDBDatabase;
 
diff --git a/dom/indexedDB/PBackgroundIDBVersionChangeTransaction.ipdl b/dom/indexedDB/PBackgroundIDBVersionChangeTransaction.ipdl
index c096c289963d..f54a9505d99b 100644
--- a/dom/indexedDB/PBackgroundIDBVersionChangeTransaction.ipdl
+++ b/dom/indexedDB/PBackgroundIDBVersionChangeTransaction.ipdl
@@ -15,7 +15,7 @@ namespace mozilla {
 namespace dom {
 namespace indexedDB {
 
-[ChildImpl="indexedDB::BackgroundVersionChangeTransactionChild", ParentImpl=virtual]
+[ChildImpl="mozilla::dom::indexedDB::BackgroundVersionChangeTransactionChild", ParentImpl=virtual]
 protocol PBackgroundIDBVersionChangeTransaction {
   manager PBackgroundIDBDatabase;
 
diff --git a/dom/indexedDB/PBackgroundIndexedDBUtils.ipdl b/dom/indexedDB/PBackgroundIndexedDBUtils.ipdl
index fbc51dea65eb..c4f143fb03c5 100644
--- a/dom/indexedDB/PBackgroundIndexedDBUtils.ipdl
+++ b/dom/indexedDB/PBackgroundIndexedDBUtils.ipdl
@@ -14,7 +14,7 @@ namespace mozilla {
 namespace dom {
 namespace indexedDB {
 
-[ManualDealloc, ChildImpl="indexedDB::BackgroundUtilsChild", ParentImpl=virtual]
+[ManualDealloc, ChildImpl="mozilla::dom::indexedDB::BackgroundUtilsChild", ParentImpl=virtual]
 sync protocol PBackgroundIndexedDBUtils
 {
   manager PBackground;
diff --git a/dom/ipc/PURLClassifierLocal.ipdl b/dom/ipc/PURLClassifierLocal.ipdl
index 34aa3ca21427..5b7c81dadbb0 100644
--- a/dom/ipc/PURLClassifierLocal.ipdl
+++ b/dom/ipc/PURLClassifierLocal.ipdl
@@ -13,7 +13,7 @@ namespace mozilla {
 namespace dom {
 
 
-[ManualDealloc, ChildImpl="URLClassifierLocalChild", ParentImpl="URLClassifierLocalParent"]
+[ManualDealloc, ChildImpl="mozilla::dom::URLClassifierLocalChild", ParentImpl="mozilla::dom::URLClassifierLocalParent"]
 protocol PURLClassifierLocal
 {
   manager PContent;
diff --git a/dom/ipc/PURLClassifierLocalByName.ipdl b/dom/ipc/PURLClassifierLocalByName.ipdl
index 372d98e5363d..5a0d64ad985f 100644
--- a/dom/ipc/PURLClassifierLocalByName.ipdl
+++ b/dom/ipc/PURLClassifierLocalByName.ipdl
@@ -14,7 +14,7 @@ include "mozilla/dom/URLClassifierChild.h";
 namespace mozilla {
 namespace dom {
 
-[ManualDealloc, ChildImpl="URLClassifierLocalByNameChild", ParentImpl="URLClassifierLocalByNameParent"]
+[ManualDealloc, ChildImpl="mozilla::dom::URLClassifierLocalByNameChild", ParentImpl="mozilla::dom::URLClassifierLocalByNameParent"]
 protocol PURLClassifierLocalByName
 {
   manager PContent;
diff --git a/dom/media/gmp/PChromiumCDM.ipdl b/dom/media/gmp/PChromiumCDM.ipdl
index 3f8f11c0f6cb..c6db95a88fbb 100644
--- a/dom/media/gmp/PChromiumCDM.ipdl
+++ b/dom/media/gmp/PChromiumCDM.ipdl
@@ -19,7 +19,7 @@ using cdm::Status from "GMPSanitizedExports.h";
 namespace mozilla {
 namespace gmp {
 
-[ChildImpl=virtual, ParentImpl="ChromiumCDMParent"]
+[ChildImpl=virtual, ParentImpl="mozilla::gmp::ChromiumCDMParent"]
 async protocol PChromiumCDM
 {
   manager PGMPContent;
diff --git a/dom/media/gmp/PGMP.ipdl b/dom/media/gmp/PGMP.ipdl
index 7df359a6635a..b5c9428c1079 100644
--- a/dom/media/gmp/PGMP.ipdl
+++ b/dom/media/gmp/PGMP.ipdl
@@ -25,7 +25,7 @@ using mozilla::dom::NativeThreadId from "mozilla/dom/NativeThreadId.h";
 namespace mozilla {
 namespace gmp {
 
-[NeedsOtherPid, NestedUpTo=inside_sync, ChildImpl="GMPChild", ParentImpl="GMPParent", ChildProc=GMPlugin]
+[NeedsOtherPid, NestedUpTo=inside_sync, ChildImpl="mozilla::gmp::GMPChild", ParentImpl="mozilla::gmp::GMPParent", ChildProc=GMPlugin]
 sync protocol PGMP
 {
   manages PGMPTimer;
diff --git a/dom/media/gmp/PGMPContent.ipdl b/dom/media/gmp/PGMPContent.ipdl
index 402cedeaaf23..d1f631a30fcf 100644
--- a/dom/media/gmp/PGMPContent.ipdl
+++ b/dom/media/gmp/PGMPContent.ipdl
@@ -15,7 +15,7 @@ include "GMPContentChild.h";
 namespace mozilla {
 namespace gmp {
 
-[NeedsOtherPid, ChildImpl="GMPContentChild", ParentImpl=virtual, ParentProc=anydom, ChildProc=GMPlugin]
+[NeedsOtherPid, ChildImpl="mozilla::gmp::GMPContentChild", ParentImpl=virtual, ParentProc=anydom, ChildProc=GMPlugin]
 sync protocol PGMPContent
 {
   manages PGMPVideoDecoder;
diff --git a/dom/media/gmp/PGMPStorage.ipdl b/dom/media/gmp/PGMPStorage.ipdl
index 5370e49ed794..d5a4c98980bb 100644
--- a/dom/media/gmp/PGMPStorage.ipdl
+++ b/dom/media/gmp/PGMPStorage.ipdl
@@ -12,7 +12,7 @@ using GMPErr from "gmp-errors.h";
 namespace mozilla {
 namespace gmp {
 
-[ChildImpl="GMPStorageChild", ParentImpl=virtual]
+[ChildImpl="mozilla::gmp::GMPStorageChild", ParentImpl=virtual]
 async protocol PGMPStorage
 {
   manager PGMP;
diff --git a/dom/media/gmp/PGMPTimer.ipdl b/dom/media/gmp/PGMPTimer.ipdl
index bef3c42739a5..c1d683c0e9ef 100644
--- a/dom/media/gmp/PGMPTimer.ipdl
+++ b/dom/media/gmp/PGMPTimer.ipdl
@@ -10,7 +10,7 @@ include "GMPTimerChild.h";
 namespace mozilla {
 namespace gmp {
 
-[ChildImpl="GMPTimerChild", ParentImpl="GMPTimerParent"]
+[ChildImpl="mozilla::gmp::GMPTimerChild", ParentImpl="mozilla::gmp::GMPTimerParent"]
 async protocol PGMPTimer
 {
   manager PGMP;
diff --git a/dom/media/gmp/PGMPVideoDecoder.ipdl b/dom/media/gmp/PGMPVideoDecoder.ipdl
index 50bc52623a4f..0b959e056e0b 100644
--- a/dom/media/gmp/PGMPVideoDecoder.ipdl
+++ b/dom/media/gmp/PGMPVideoDecoder.ipdl
@@ -14,7 +14,7 @@ include "GMPVideoDecoderChild.h";
 namespace mozilla {
 namespace gmp {
 
-[ChildImpl="GMPVideoDecoderChild", ParentImpl=virtual]
+[ChildImpl="mozilla::gmp::GMPVideoDecoderChild", ParentImpl=virtual]
 async protocol PGMPVideoDecoder
 {
   manager PGMPContent;
diff --git a/dom/media/gmp/PGMPVideoEncoder.ipdl b/dom/media/gmp/PGMPVideoEncoder.ipdl
index 753041f0fa0a..a89b8f3871c6 100644
--- a/dom/media/gmp/PGMPVideoEncoder.ipdl
+++ b/dom/media/gmp/PGMPVideoEncoder.ipdl
@@ -15,7 +15,7 @@ include "GMPVideoEncoderChild.h";
 namespace mozilla {
 namespace gmp {
 
-[ChildImpl="GMPVideoEncoderChild", ParentImpl=virtual]
+[ChildImpl="mozilla::gmp::GMPVideoEncoderChild", ParentImpl=virtual]
 async protocol PGMPVideoEncoder
 {
   manager PGMPContent;
diff --git a/dom/media/ipc/PRemoteEncoder.ipdl b/dom/media/ipc/PRemoteEncoder.ipdl
index 503f9001d53a..67f176ebc88b 100644
--- a/dom/media/ipc/PRemoteEncoder.ipdl
+++ b/dom/media/ipc/PRemoteEncoder.ipdl
@@ -54,7 +54,7 @@ union EncodeResultIPDL
 // The child side runs in the content process, and the parent side runs
 // in the RDD/utility/GPU processes. We run a separate IPDL thread for both
 // sides.
-[ChildImpl="RemoteMediaDataEncoderChild", ParentImpl="RemoteMediaDataEncoderParent"]
+[ChildImpl="mozilla::RemoteMediaDataEncoderChild", ParentImpl="mozilla::RemoteMediaDataEncoderParent"]
 async protocol PRemoteEncoder
 {
   manager PRemoteMediaManager;
diff --git a/dom/media/systemservices/PMedia.ipdl b/dom/media/systemservices/PMedia.ipdl
index c3d8476b5ff4..bb2fe2694adf 100644
--- a/dom/media/systemservices/PMedia.ipdl
+++ b/dom/media/systemservices/PMedia.ipdl
@@ -11,7 +11,7 @@ include "mozilla/media/MediaChild.h";
 namespace mozilla {
 namespace media {
 
-[ManualDealloc, ChildImpl="Child", ParentImpl=virtual]
+[ManualDealloc, ChildImpl="mozilla::media::Child", ParentImpl=virtual]
 protocol PMedia
 {
   manager PContent;
diff --git a/dom/media/webrtc/PMediaTransport.ipdl b/dom/media/webrtc/PMediaTransport.ipdl
index 44bb4ae7d51f..1feb9f552220 100644
--- a/dom/media/webrtc/PMediaTransport.ipdl
+++ b/dom/media/webrtc/PMediaTransport.ipdl
@@ -4,6 +4,9 @@
 
 include protocol PBackground;
 
+include "mozilla/dom/MediaTransportChild.h";
+include "mozilla/dom/MediaTransportParent.h";
+
 #ifdef MOZ_WEBRTC
 include WebrtcProxyConfig;
 
@@ -50,7 +53,7 @@ struct IceCandidateAttributePair {
 };
 #endif // MOZ_WEBRTC
 
-[ParentProc=Socket, ChildProc=Content]
+[ParentProc=Socket, ChildProc=Content, ParentImpl="mozilla::MediaTransportParent", ChildImpl="mozilla::MediaTransportChild"]
 async protocol PMediaTransport {
 #ifdef MOZ_WEBRTC
 parent:
diff --git a/dom/storage/PBackgroundSessionStorageService.ipdl b/dom/storage/PBackgroundSessionStorageService.ipdl
index ef6087f0c3c1..e5518ddb2418 100644
--- a/dom/storage/PBackgroundSessionStorageService.ipdl
+++ b/dom/storage/PBackgroundSessionStorageService.ipdl
@@ -7,7 +7,7 @@ include protocol PBackground;
 namespace mozilla {
 namespace dom {
 
-[ChildImpl="SessionStorageService"]
+[ChildImpl="mozilla::dom::SessionStorageService"]
 protocol PBackgroundSessionStorageService
 {
   manager PBackground;
diff --git a/extensions/spellcheck/ipc/PRemoteSpellcheckEngine.ipdl b/extensions/spellcheck/ipc/PRemoteSpellcheckEngine.ipdl
index c327d97e65a1..e979203dc019 100644
--- a/extensions/spellcheck/ipc/PRemoteSpellcheckEngine.ipdl
+++ b/extensions/spellcheck/ipc/PRemoteSpellcheckEngine.ipdl
@@ -9,7 +9,7 @@ include "mozilla/RemoteSpellCheckEngineChild.h";
 
 namespace mozilla {
 
-[ManualDealloc, ChildImpl="RemoteSpellcheckEngineChild", ParentImpl="RemoteSpellcheckEngineParent"]
+[ManualDealloc, ChildImpl="mozilla::RemoteSpellcheckEngineChild", ParentImpl="mozilla::RemoteSpellcheckEngineParent"]
 sync protocol PRemoteSpellcheckEngine {
   manager PContent;
 
diff --git a/gfx/layers/ipc/PAPZ.ipdl b/gfx/layers/ipc/PAPZ.ipdl
index 1c6b357f1d38..8c972dfbd7f1 100644
--- a/gfx/layers/ipc/PAPZ.ipdl
+++ b/gfx/layers/ipc/PAPZ.ipdl
@@ -39,7 +39,7 @@ namespace layers {
  * are implemented. If a new method is needed then PAPZ, APZChild, and RemoteContentController
  * must be updated to handle it.
  */
-[ParentImpl="RemoteContentController"]
+[ParentImpl="mozilla::layers::RemoteContentController"]
 sync protocol PAPZ
 {
   manager PCompositorBridge;
diff --git a/gfx/layers/ipc/PCanvas.ipdl b/gfx/layers/ipc/PCanvas.ipdl
index c6b4fed16ce2..88f553b0f9a4 100644
--- a/gfx/layers/ipc/PCanvas.ipdl
+++ b/gfx/layers/ipc/PCanvas.ipdl
@@ -20,7 +20,7 @@ namespace layers {
 /**
  * PCanvas is the IPDL for recorded Canvas drawing.
  */
-[ParentImpl="CanvasTranslator"]
+[ParentImpl="mozilla::layers::CanvasTranslator"]
 async protocol PCanvas {
   manager PCanvasManager;
 
diff --git a/gfx/layers/ipc/PCompositorBridge.ipdl b/gfx/layers/ipc/PCompositorBridge.ipdl
index 225651dd7b73..56cf8e512a73 100644
--- a/gfx/layers/ipc/PCompositorBridge.ipdl
+++ b/gfx/layers/ipc/PCompositorBridge.ipdl
@@ -82,7 +82,7 @@ struct FrameStats {
  * protocol, which is per-browser. A lot of the functions in the protocol are
  * basically multiplexing/demultiplexing stuff in PLayerTransaction.
  */
-[ParentImpl="CompositorBridgeParentBase"]
+[ParentImpl="mozilla::layers::CompositorBridgeParentBase"]
 sync protocol PCompositorBridge
 {
   manager PCompositorManager;
diff --git a/gfx/vr/ipc/PVR.ipdl b/gfx/vr/ipc/PVR.ipdl
index 74574febf07d..4be8870ca088 100644
--- a/gfx/vr/ipc/PVR.ipdl
+++ b/gfx/vr/ipc/PVR.ipdl
@@ -18,7 +18,7 @@ include protocol PVRGPU;
 namespace mozilla {
 namespace gfx {
 
-[NeedsOtherPid, ChildImpl="VRChild", ParentImpl="VRParent", ParentProc=VR, ChildProc=Parent]
+[NeedsOtherPid, ChildImpl="mozilla::gfx::VRChild", ParentImpl="mozilla::gfx::VRParent", ParentProc=VR, ChildProc=Parent]
 async protocol PVR
 {
 parent:
diff --git a/gfx/vr/ipc/PVRGPU.ipdl b/gfx/vr/ipc/PVRGPU.ipdl
index 783515e66053..3c99fca3ff49 100644
--- a/gfx/vr/ipc/PVRGPU.ipdl
+++ b/gfx/vr/ipc/PVRGPU.ipdl
@@ -10,7 +10,7 @@ namespace gfx {
 
 // The parent process is the VR process.
 // The child process is the GPU process.
-[NeedsOtherPid, ChildImpl="VRGPUChild", ParentImpl="VRGPUParent", ParentProc=VR, ChildProc=GPU]
+[NeedsOtherPid, ChildImpl="mozilla::gfx::VRGPUChild", ParentImpl="mozilla::gfx::VRGPUParent", ParentProc=VR, ChildProc=GPU]
 async protocol PVRGPU
 {
 parent:
@@ -24,4 +24,4 @@ child:
 };
 
 } // gfx
-} // mozilla
\ No newline at end of file
+} // mozilla
diff --git a/gfx/vr/ipc/PVRLayer.ipdl b/gfx/vr/ipc/PVRLayer.ipdl
index c96a2a7542a0..80a2294f1d1e 100644
--- a/gfx/vr/ipc/PVRLayer.ipdl
+++ b/gfx/vr/ipc/PVRLayer.ipdl
@@ -13,7 +13,7 @@ using mozilla::gfx::Rect from "mozilla/gfx/Rect.h";
 namespace mozilla {
 namespace gfx {
 
-[ChildImpl="VRLayerChild", ParentImpl=virtual]
+[ChildImpl="mozilla::gfx::VRLayerChild", ParentImpl=virtual]
 async protocol PVRLayer
 {
   manager PVRManager;
diff --git a/gfx/vr/ipc/PVRManager.ipdl b/gfx/vr/ipc/PVRManager.ipdl
index 9bfc94276352..b4f01ac49783 100644
--- a/gfx/vr/ipc/PVRManager.ipdl
+++ b/gfx/vr/ipc/PVRManager.ipdl
@@ -31,7 +31,7 @@ namespace gfx {
  * enumeration and sensor state between the compositor thread and
  * content threads/processes.
  */
-[NeedsOtherPid, ChildImpl="VRManagerChild", ParentImpl="VRManagerParent", ParentProc=compositor, ChildProc=anydom]
+[NeedsOtherPid, ChildImpl="mozilla::gfx::VRManagerChild", ParentImpl="mozilla::gfx::VRManagerParent", ParentProc=compositor, ChildProc=anydom]
 sync protocol PVRManager
 {
   manages PVRLayer;
diff --git a/ipc/ipdl/ipdl/lower.py b/ipc/ipdl/ipdl/lower.py
index 006ca50ee8e1..57a6970a7962 100644
--- a/ipc/ipdl/ipdl/lower.py
+++ b/ipc/ipdl/ipdl/lower.py
@@ -3246,6 +3246,7 @@ class _GenerateProtocolActorCode(ipdl.ast.Visitor):
         self.cppfile = None
         self.ns = None
         self.cls = None
+        self.concreteActorType = None
         self.protocolCxxIncludes = []
         self.actorForwardDecls = []
         self.usingDecls = []
@@ -3356,18 +3357,6 @@ class _GenerateProtocolActorCode(ipdl.ast.Visitor):
 
         self.hdrfile.addthings([traitsdecl, Whitespace.NL] + _includeGuardEnd(hf))
 
-        # If the implementation type is not overridden, add an implicit import
-        # for the default implementation header file. Explicit implementation
-        # types will specify their headers manually with `include`.
-        if self.protocol.implAttribute(self.side) is None:
-            assert self.protocol.name.startswith("P")
-            self.externalIncludes.add(
-                "".join(n.name + "/" for n in self.protocol.namespaces)
-                + self.protocol.name[1:]
-                + self.side.capitalize()
-                + ".h"
-            )
-
         # make the .cpp file
         cf.addthings(
             [
@@ -3520,6 +3509,30 @@ class _GenerateProtocolActorCode(ipdl.ast.Visitor):
                 Whitespace.NL,
             ])
 
+        implAttr = self.protocol.implAttribute(self.side)
+        if implAttr is None:
+            assert self.protocol.name.startswith("P")
+            pqname = self.protocol.qname()
+            self.concreteActorType = ipdl.ast.QualifiedId(
+                pqname.loc, pqname.baseid[1:] + self.side.title(), pqname.quals
+            )
+
+            # If the implementation type is not overridden, add an implicit
+            # import for the default implementation header file. Explicit
+            # implementation types will specify their headers manually with
+            # `include`.
+            self.externalIncludes.add(
+                "/".join(
+                    self.concreteActorType.quals
+                    + [self.concreteActorType.baseid + ".h"]
+                )
+            )
+        elif implAttr != "virtual":
+            parts = implAttr.value.split("::")
+            self.concreteActorType = ipdl.ast.QualifiedId(
+                implAttr.loc, parts[-1], parts[:-1]
+            )
+
         self.cls = Class(self.clsname, inherits=inherits, abstract=True)
 
         self.cls.addstmt(Label.PRIVATE)
@@ -3604,7 +3617,7 @@ class _GenerateProtocolActorCode(ipdl.ast.Visitor):
                     defaultRecv = MethodDefn(recvDecl)
                     defaultRecv.addcode("return IPC_OK();\n")
                     self.cls.addstmt(defaultRecv)
-                elif self.protocol.implAttribute(self.side) == "virtual":
+                elif self.concreteActorType is None:
                     # If we're using virtual calls, we need the methods to be
                     # declared on the base class.
                     recvDecl.methodspec = MethodSpec.PURE
@@ -3612,7 +3625,7 @@ class _GenerateProtocolActorCode(ipdl.ast.Visitor):
 
         # If we're using virtual calls, we need the methods to be declared on
         # the base class.
-        if self.protocol.implAttribute(self.side) == "virtual":
+        if self.concreteActorType is None:
             for md in p.messageDecls:
                 managed = md.decl.type.constructedType()
                 if not ptype.isManagerOf(managed) or md.decl.type.isDtor():
@@ -4287,18 +4300,11 @@ class _GenerateProtocolActorCode(ipdl.ast.Visitor):
     ##
 
     def concreteThis(self):
-        implAttr = self.protocol.implAttribute(self.side)
-        if implAttr == "virtual":
+        if self.concreteActorType is None:
             return ExprVar.THIS
-
-        if implAttr is None:
-            assert self.protocol.name.startswith("P")
-            className = self.protocol.name[1:] + self.side.capitalize()
-        else:
-            assert isinstance(implAttr, ipdl.ast.StringLiteral)
-            className = implAttr.value
-
-        return ExprCode("static_cast<${className}*>(this)", className=className)
+        return ExprCast(
+            ExprVar.THIS, Type(str(self.concreteActorType), ptr=True), static=True
+        )
 
     def thisCall(self, function, args):
         return ExprCall(ExprSelect(self.concreteThis(), "->", function), args=args)
diff --git a/ipc/testshell/PTestShellCommand.ipdl b/ipc/testshell/PTestShellCommand.ipdl
index 9c02b7c4c9e5..fb5d8b4c89d9 100644
--- a/ipc/testshell/PTestShellCommand.ipdl
+++ b/ipc/testshell/PTestShellCommand.ipdl
@@ -9,7 +9,7 @@ include "mozilla/ipc/TestShellParent.h";
 namespace mozilla {
 namespace ipc {
 
-[ManualDealloc, ChildImpl=virtual, ParentImpl="TestShellCommandParent"]
+[ManualDealloc, ChildImpl=virtual, ParentImpl="mozilla::ipc::TestShellCommandParent"]
 protocol PTestShellCommand
 {
   manager PTestShell;
diff --git a/js/xpconnect/loader/PScriptCache.ipdl b/js/xpconnect/loader/PScriptCache.ipdl
index 3e11bfdae34c..7ec24e891b61 100644
--- a/js/xpconnect/loader/PScriptCache.ipdl
+++ b/js/xpconnect/loader/PScriptCache.ipdl
@@ -21,7 +21,7 @@ struct ScriptData {
     uint8_t[] xdrData;
 };
 
-[ManualDealloc, ChildImpl="ScriptCacheChild", ParentImpl="ScriptCacheParent"]
+[ManualDealloc, ChildImpl="mozilla::loader::ScriptCacheChild", ParentImpl="mozilla::loader::ScriptCacheParent"]
 protocol PScriptCache
 {
     manager PContent;
diff --git a/netwerk/protocol/websocket/PTransportProvider.ipdl b/netwerk/protocol/websocket/PTransportProvider.ipdl
index 26885c911c43..5da9fcbd31d0 100644
--- a/netwerk/protocol/websocket/PTransportProvider.ipdl
+++ b/netwerk/protocol/websocket/PTransportProvider.ipdl
@@ -15,7 +15,7 @@ namespace net {
  * again. Hence there's no need for any messages on the protocol itself.
  */
 
-[ManualDealloc, ChildImpl="TransportProviderChild", ParentImpl="TransportProviderParent"]
+[ManualDealloc, ChildImpl="mozilla::net::TransportProviderChild", ParentImpl="mozilla::net::TransportProviderParent"]
 async protocol PTransportProvider
 {
   manager PNecko;
diff --git a/netwerk/protocol/websocket/PWebSocket.ipdl b/netwerk/protocol/websocket/PWebSocket.ipdl
index dbe764be87f1..91cba338733f 100644
--- a/netwerk/protocol/websocket/PWebSocket.ipdl
+++ b/netwerk/protocol/websocket/PWebSocket.ipdl
@@ -17,7 +17,7 @@ using class IPC::SerializedLoadContext from "SerializedLoadContext.h";
 namespace mozilla {
 namespace net {
 
-[ManualDealloc, ChildImpl="WebSocketChannelChild", ParentImpl="WebSocketChannelParent"]
+[ManualDealloc, ChildImpl="mozilla::net::WebSocketChannelChild", ParentImpl="mozilla::net::WebSocketChannelParent"]
 async protocol PWebSocket
 {
   manager PNecko;