diff --git a/ipc/glue/MessageChannel.cpp b/ipc/glue/MessageChannel.cpp
index 1f3955597d96..653b38ff79a9 100644
--- a/ipc/glue/MessageChannel.cpp
+++ b/ipc/glue/MessageChannel.cpp
@@ -1628,7 +1628,14 @@ MessageChannel::MessageTask::Post()
     mScheduled = true;
 
     RefPtr<MessageTask> self = this;
-    mChannel->mWorkerLoop->PostTask(self.forget());
+    nsCOMPtr<nsIEventTarget> eventTarget =
+        mChannel->mListener->GetMessageEventTarget(mMessage);
+
+    if (eventTarget) {
+        eventTarget->Dispatch(self.forget(), NS_DISPATCH_NORMAL);
+    } else {
+        mChannel->mWorkerLoop->PostTask(self.forget());
+    }
 }
 
 void
diff --git a/ipc/glue/ProtocolUtils.cpp b/ipc/glue/ProtocolUtils.cpp
index 9933a28b2274..8e076e37a512 100644
--- a/ipc/glue/ProtocolUtils.cpp
+++ b/ipc/glue/ProtocolUtils.cpp
@@ -417,6 +417,9 @@ IProtocol::Lookup(int32_t aId)
 void
 IProtocol::Unregister(int32_t aId)
 {
+  if (mId == aId) {
+    mId = kFreedActorId;
+  }
   Manager()->Unregister(aId);
 }
 
@@ -518,12 +521,35 @@ IProtocol::DeallocShmem(Shmem& aMem)
   return ok;
 }
 
+void
+IProtocol::SetManager(IProtocol* aManager)
+{
+  MOZ_RELEASE_ASSERT(!mManager || mManager == aManager);
+  mManager = aManager;
+}
+
+void
+IProtocol::SetEventTargetForActor(IProtocol* aActor, nsIEventTarget* aEventTarget)
+{
+  // Make sure we have a manager for the internal method to access.
+  aActor->SetManager(this);
+  SetEventTargetForActorInternal(aActor, aEventTarget);
+}
+
+void
+IProtocol::SetEventTargetForActorInternal(IProtocol* aActor,
+                                          nsIEventTarget* aEventTarget)
+{
+  Manager()->SetEventTargetForActorInternal(aActor, aEventTarget);
+}
+
 IToplevelProtocol::IToplevelProtocol(ProtocolId aProtoId, Side aSide)
  : IProtocol(aSide),
    mProtocolId(aProtoId),
    mOtherPid(mozilla::ipc::kInvalidProcessId),
-   mLastRouteId(aSide == ParentSide ? 1 : 0),
-   mLastShmemId(aSide == ParentSide ? 1 : 0)
+   mLastRouteId(aSide == ParentSide ? kFreedActorId : kNullActorId),
+   mLastShmemId(aSide == ParentSide ? kFreedActorId : kNullActorId),
+   mEventTargetMutex("ProtocolEventTargetMutex")
 {
 }
 
@@ -598,8 +624,22 @@ IToplevelProtocol::IsOnCxxStack() const
 int32_t
 IToplevelProtocol::Register(IProtocol* aRouted)
 {
+  if (aRouted->Id() != kNullActorId && aRouted->Id() != kFreedActorId) {
+    // If there's already an ID, just return that.
+    return aRouted->Id();
+  }
   int32_t id = GetSide() == ParentSide ? ++mLastRouteId : --mLastRouteId;
   mActorMap.AddWithID(aRouted, id);
+  aRouted->SetId(id);
+
+  // Inherit our event target from our manager.
+  if (IProtocol* manager = aRouted->Manager()) {
+    MutexAutoLock lock(mEventTargetMutex);
+    if (nsCOMPtr<nsIEventTarget> target = mEventTargetMap.Lookup(manager->Id())) {
+      mEventTargetMap.AddWithID(target, id);
+    }
+  }
+
   return id;
 }
 
@@ -608,6 +648,7 @@ IToplevelProtocol::RegisterID(IProtocol* aRouted,
                               int32_t aId)
 {
   mActorMap.AddWithID(aRouted, aId);
+  aRouted->SetId(aId);
   return aId;
 }
 
@@ -620,7 +661,10 @@ IToplevelProtocol::Lookup(int32_t aId)
 void
 IToplevelProtocol::Unregister(int32_t aId)
 {
-  return mActorMap.Remove(aId);
+  mActorMap.Remove(aId);
+
+  MutexAutoLock lock(mEventTargetMutex);
+  mEventTargetMap.RemoveIfPresent(aId);
 }
 
 Shmem::SharedMemory*
@@ -726,5 +770,52 @@ IToplevelProtocol::ShmemDestroyed(const Message& aMsg)
   return true;
 }
 
+already_AddRefed<nsIEventTarget>
+IToplevelProtocol::GetMessageEventTarget(const Message& aMsg)
+{
+  int32_t route = aMsg.routing_id();
+
+  MutexAutoLock lock(mEventTargetMutex);
+  nsCOMPtr<nsIEventTarget> target = mEventTargetMap.Lookup(route);
+
+  if (aMsg.is_constructor()) {
+    ActorHandle handle;
+    PickleIterator iter = PickleIterator(aMsg);
+    if (!IPC::ReadParam(&aMsg, &iter, &handle)) {
+      return nullptr;
+    }
+
+    // Normally a new actor inherits its event target from its manager. If the
+    // manager has no event target, we give the subclass a chance to make a new
+    // one.
+    if (!target) {
+      MutexAutoUnlock unlock(mEventTargetMutex);
+      target = GetConstructedEventTarget(aMsg);
+    }
+
+    mEventTargetMap.AddWithID(target, handle.mId);
+  }
+
+  return target.forget();
+}
+
+void
+IToplevelProtocol::SetEventTargetForActorInternal(IProtocol* aActor,
+                                                  nsIEventTarget* aEventTarget)
+{
+  // We should only call this function on actors that haven't been used for IPC
+  // code yet. Otherwise we'll be posting stuff to the wrong event target before
+  // we're called.
+  MOZ_RELEASE_ASSERT(aActor->Id() == kNullActorId || aActor->Id() == kFreedActorId);
+
+  // Register the actor early. When it's registered again, it will keep the same
+  // ID.
+  int32_t id = Register(aActor);
+  aActor->SetId(id);
+
+  MutexAutoLock lock(mEventTargetMutex);
+  mEventTargetMap.AddWithID(aEventTarget, id);
+}
+
 } // namespace ipc
 } // namespace mozilla
diff --git a/ipc/glue/ProtocolUtils.h b/ipc/glue/ProtocolUtils.h
index 3bc9e0c74480..3e5c2c6d195e 100644
--- a/ipc/glue/ProtocolUtils.h
+++ b/ipc/glue/ProtocolUtils.h
@@ -16,6 +16,7 @@
 #include "prenv.h"
 
 #include "IPCMessageStart.h"
+#include "mozilla/AlreadyAddRefed.h"
 #include "mozilla/Attributes.h"
 #include "mozilla/ipc/FileDescriptor.h"
 #include "mozilla/ipc/Shmem.h"
@@ -56,6 +57,8 @@ enum {
 
 } // namespace
 
+class nsIEventTarget;
+
 namespace mozilla {
 namespace dom {
 class ContentParent;
@@ -141,6 +144,8 @@ enum RacyInterruptPolicy {
     RIPParentWins
 };
 
+class IToplevelProtocol;
+
 class IProtocol : public HasResultCodes
 {
 public:
@@ -196,11 +201,26 @@ class IProtocol : public HasResultCodes
     bool AllocUnsafeShmem(size_t aSize, Shmem::SharedMemory::SharedMemoryType aType, Shmem* aOutMem);
     bool DeallocShmem(Shmem& aMem);
 
+    // Sets an event target to which all messages for aActor will be
+    // dispatched. This method must be called before right before the SendPFoo
+    // message for aActor is sent. And SendPFoo *must* be called if
+    // SetEventTargetForActor is called. The receiver when calling
+    // SetEventTargetForActor must be the actor that will be the manager for
+    // aActor.
+    void SetEventTargetForActor(IProtocol* aActor, nsIEventTarget* aEventTarget);
+
 protected:
+    friend class IToplevelProtocol;
+
     void SetId(int32_t aId) { mId = aId; }
-    void SetManager(IProtocol* aManager) { mManager = aManager; }
+    void SetManager(IProtocol* aManager);
     void SetIPCChannel(MessageChannel* aChannel) { mChannel = aChannel; }
 
+    virtual void SetEventTargetForActorInternal(IProtocol* aActor, nsIEventTarget* aEventTarget);
+
+    static const int32_t kNullActorId = 0;
+    static const int32_t kFreedActorId = 1;
+
 private:
     int32_t mId;
     Side mSide;
@@ -359,7 +379,16 @@ class IToplevelProtocol : public IProtocol
     virtual void ProcessRemoteNativeEventsInInterruptCall() {
     }
 
-private:
+    virtual already_AddRefed<nsIEventTarget>
+    GetMessageEventTarget(const Message& aMsg);
+
+protected:
+    virtual already_AddRefed<nsIEventTarget>
+    GetConstructedEventTarget(const Message& aMsg) { return nullptr; }
+
+    virtual void SetEventTargetForActorInternal(IProtocol* aActor, nsIEventTarget* aEventTarget);
+
+  private:
     ProtocolId mProtocolId;
     UniquePtr<Transport> mTrans;
     base::ProcessId mOtherPid;
@@ -367,6 +396,9 @@ class IToplevelProtocol : public IProtocol
     int32_t mLastRouteId;
     IDMap<Shmem::SharedMemory*> mShmemMap;
     Shmem::id_t mLastShmemId;
+
+    Mutex mEventTargetMutex;
+    IDMap<nsCOMPtr<nsIEventTarget>> mEventTargetMap;
 };
 
 class IShmemAllocator
diff --git a/ipc/ipdl/ipdl/lower.py b/ipc/ipdl/ipdl/lower.py
index b81f370aec5e..9f0430aabb4f 100644
--- a/ipc/ipdl/ipdl/lower.py
+++ b/ipc/ipdl/ipdl/lower.py
@@ -4096,15 +4096,15 @@ def ctorPrologue(self, md, errfn=ExprLiteral.NULL, idexpr=None):
         actortype = ipdl.type.ActorType(actorproto)
 
         if idexpr is None:
-            idexpr = ExprCall(self.protocol.registerMethod(),
-                              args=[ actorvar ])
+            registerexpr = ExprCall(self.protocol.registerMethod(),
+                                    args=[ actorvar ])
         else:
-            idexpr = ExprCall(self.protocol.registerIDMethod(),
-                              args=[ actorvar, idexpr ])
+            registerexpr = ExprCall(self.protocol.registerIDMethod(),
+                                    args=[ actorvar, idexpr ])
 
         return [
             self.failIfNullActor(actorvar, errfn, msg="Error constructing actor %s" % actortype.name() + self.side.capitalize()),
-            StmtExpr(ExprCall(ExprSelect(actorvar, '->', 'SetId'), args=[idexpr])),
+            StmtExpr(ExprCall(registerexpr)),
             StmtExpr(ExprCall(ExprSelect(actorvar, '->', 'SetManager'), args=[ExprVar.THIS])),
             StmtExpr(ExprCall(ExprSelect(actorvar, '->', 'SetIPCChannel'),
                               args=[self.protocol.callGetChannel()])),
@@ -4211,11 +4211,14 @@ def destroyActor(self, md, actorexpr, why=_DestroyReason.Deletion):
             destroyedType = md.decl.type.constructedType()
         else:
             destroyedType = self.protocol.decl.type
-        return ([ StmtExpr(self.callActorDestroy(actorexpr, why)),
+        managervar = ExprVar('mgr')
+        return ([ StmtDecl(Decl(Type('IProtocol', ptr=1), managervar.name),
+                           init=self.protocol.managerVar(actorexpr)),
+                  StmtExpr(self.callActorDestroy(actorexpr, why)),
                   StmtExpr(self.callDeallocSubtree(md, actorexpr)),
                   StmtExpr(self.callRemoveActor(
                       actorexpr,
-                      manager=self.protocol.managerVar(actorexpr),
+                      manager=managervar,
                       ipdltype=destroyedType))
                 ])
 
@@ -4358,8 +4361,7 @@ def failIfNullActor(self, actorExpr, retOnNull=ExprLiteral.FALSE, msg=None):
 
     def unregisterActor(self):
         return [ StmtExpr(ExprCall(self.protocol.unregisterMethod(),
-                                   args=[ _actorId() ])),
-                 StmtExpr(ExprCall(ExprVar('SetId'), args=[_FREED_ACTOR_ID])) ]
+                                   args=[ _actorId() ])) ]
 
     def makeMessage(self, md, errfn, fromActor=None):
         msgvar = self.msgvar