--- a/xpcom/threads/SyncRunnable.h
+++ b/xpcom/threads/SyncRunnable.h
@@ -28,16 +28,17 @@ namespace mozilla {
  *
  */
 class SyncRunnable : public nsRunnable
 {
 public:
   SyncRunnable(nsIRunnable* r)
     : mRunnable(r)
     , mMonitor("SyncRunnable")
+    , mDone(false)
   { }
 
   void DispatchToThread(nsIEventTarget* thread,
                         bool forceDispatch = false)
   {
     nsresult rv;
     bool on;
 
@@ -45,39 +46,48 @@ public:
       rv = thread->IsOnCurrentThread(&on);
       MOZ_ASSERT(NS_SUCCEEDED(rv));
       if (NS_SUCCEEDED(rv) && on) {
         mRunnable->Run();
         return;
       }
     }
 
-    mozilla::MonitorAutoLock lock(mMonitor);
     rv = thread->Dispatch(this, NS_DISPATCH_NORMAL);
     if (NS_SUCCEEDED(rv)) {
-      lock.Wait();
+      mozilla::MonitorAutoLock lock(mMonitor);
+      while (!mDone) {
+        lock.Wait();
+      }
     }
   }
 
   static void DispatchToThread(nsIEventTarget* thread,
                                nsIRunnable* r,
                                bool forceDispatch = false)
   {
     nsRefPtr<SyncRunnable> s(new SyncRunnable(r));
     s->DispatchToThread(thread, forceDispatch);
   }
 
 protected:
   NS_IMETHODIMP Run()
   {
     mRunnable->Run();
-    mozilla::MonitorAutoLock(mMonitor).Notify();
+
+    mozilla::MonitorAutoLock lock(mMonitor);
+    MOZ_ASSERT(!mDone);
+
+    mDone = true;
+    mMonitor.Notify();
+
     return NS_OK;
   }
 
 private:
   nsCOMPtr<nsIRunnable> mRunnable;
   mozilla::Monitor mMonitor;
+  bool mDone;
 };
 
 } // namespace mozilla
 
 #endif // mozilla_SyncRunnable_h