--- a/dom/messagechannel/MessagePort.cpp
+++ b/dom/messagechannel/MessagePort.cpp
@@ -559,16 +559,23 @@ MessagePort::Dispatch()
       break;
   }
 
   RefPtr<SharedMessagePortMessage> data = mMessages.ElementAt(0);
   mMessages.RemoveElementAt(0);
 
   mPostMessageRunnable = new PostMessageRunnable(this, data);
 
+  if (NS_IsMainThread()) {
+    if (nsCOMPtr<nsIGlobalObject> global = GetOwnerGlobal()) {
+      MOZ_ALWAYS_SUCCEEDS(global->Dispatch("MessagePortMessage", TaskCategory::Other, do_AddRef(mPostMessageRunnable)));
+      return;
+    }
+  }
+
   MOZ_ALWAYS_SUCCEEDS(NS_DispatchToCurrentThread(mPostMessageRunnable));
 }
 
 void
 MessagePort::Close()
 {
   CloseInternal(true /* aSoftly */);
 }