--- a/xpcom/threads/nsThreadPool.cpp
+++ b/xpcom/threads/nsThreadPool.cpp
@@ -267,17 +267,18 @@ nsThreadPool::Dispatch(already_AddRefed<
     RefPtr<nsThreadSyncDispatch> wrapper =
       new nsThreadSyncDispatch(thread, Move(aEvent));
     PutEvent(wrapper);
 
     while (wrapper->IsPending()) {
       NS_ProcessNextEvent(thread);
     }
   } else {
-    NS_ASSERTION(aFlags == NS_DISPATCH_NORMAL, "unexpected dispatch flags");
+    NS_ASSERTION(aFlags == NS_DISPATCH_NORMAL ||
+                 aFlags == NS_DISPATCH_TAIL, "unexpected dispatch flags");
     PutEvent(Move(aEvent), aFlags);
   }
   return NS_OK;
 }
 
 NS_IMETHODIMP
 nsThreadPool::IsOnCurrentThread(bool* aResult)
 {