--- a/xpcom/threads/nsThread.cpp
+++ b/xpcom/threads/nsThread.cpp
@@ -421,17 +421,17 @@ nsThread::Dispatch(nsIRunnable *event, P
       return NS_ERROR_OUT_OF_MEMORY;
     nsresult rv = PutEvent(wrapper);
     // Don't wait for the event to finish if we didn't dispatch it...
     if (NS_FAILED(rv))
       return rv;
 
     while (wrapper->IsPending())
       NS_ProcessNextEvent(thread);
-    return rv;
+    return wrapper->Result();
   }
 
   NS_ASSERTION(flags == NS_DISPATCH_NORMAL, "unexpected dispatch flags");
   return PutEvent(event);
 }
 
 NS_IMETHODIMP
 nsThread::IsOnCurrentThread(PRBool *result)
@@ -750,17 +750,17 @@ nsThread::GetRecursionDepth(PRUint32 *de
 }
 
 //-----------------------------------------------------------------------------
 
 NS_IMETHODIMP
 nsThreadSyncDispatch::Run()
 {
   if (mSyncTask) {
-    mSyncTask->Run();
+    mResult = mSyncTask->Run();
     mSyncTask = nsnull;
     // unblock the origin thread
     mOrigin->Dispatch(this, NS_DISPATCH_NORMAL);
   }
   return NS_OK;
 }
 
 nsresult

--- a/xpcom/threads/nsThread.h
+++ b/xpcom/threads/nsThread.h
@@ -149,23 +149,28 @@ private:
   PRPackedBool mEventsAreDoomed;
 };
 
 //-----------------------------------------------------------------------------
 
 class nsThreadSyncDispatch : public nsRunnable {
 public:
   nsThreadSyncDispatch(nsIThread *origin, nsIRunnable *task)
-    : mOrigin(origin), mSyncTask(task) {
+    : mOrigin(origin), mSyncTask(task), mResult(NS_ERROR_NOT_INITIALIZED) {
   }
 
   PRBool IsPending() {
     return mSyncTask != nsnull;
   }
 
+  nsresult Result() {
+    return mResult;
+  }
+
 private:
   NS_DECL_NSIRUNNABLE
 
   nsCOMPtr<nsIThread> mOrigin;
   nsCOMPtr<nsIRunnable> mSyncTask;
+  nsresult mResult;
 };
 
 #endif  // nsThread_h__