--- a/xpcom/threads/TaskQueue.cpp
+++ b/xpcom/threads/TaskQueue.cpp
@@ -40,29 +40,30 @@ TaskQueue::TailDispatcher()
 }
 
 nsresult
 TaskQueue::DispatchLocked(already_AddRefed<nsIRunnable> aRunnable,
                                DispatchMode aMode, DispatchFailureHandling aFailureHandling,
                                DispatchReason aReason)
 {
   nsCOMPtr<nsIRunnable> r = aRunnable;
-  AbstractThread* currentThread;
-  if (aReason != TailDispatch && (currentThread = GetCurrent()) && RequiresTailDispatch(currentThread)) {
-    currentThread->TailDispatcher().AddTask(this, r.forget(), aFailureHandling);
-    return NS_OK;
-  }
-
   mQueueMonitor.AssertCurrentThreadOwns();
   if (mIsFlushing && aMode == AbortIfFlushing) {
     return NS_ERROR_ABORT;
   }
   if (mIsShutdown) {
     return NS_ERROR_FAILURE;
   }
+
+  AbstractThread* currentThread;
+  if (aReason != TailDispatch && (currentThread = GetCurrent()) && RequiresTailDispatch(currentThread)) {
+    currentThread->TailDispatcher().AddTask(this, r.forget(), aFailureHandling);
+    return NS_OK;
+  }
+
   mTasks.push(r.forget());
   if (mIsRunning) {
     return NS_OK;
   }
   RefPtr<nsIRunnable> runner(new Runner(this));
   nsresult rv = mPool->Dispatch(runner.forget(), NS_DISPATCH_NORMAL);
   if (NS_FAILED(rv)) {
     NS_WARNING("Failed to dispatch runnable to run TaskQueue");