--- a/xpcom/threads/nsThread.cpp
+++ b/xpcom/threads/nsThread.cpp
@@ -977,16 +977,22 @@ nsThread::RegisterIdlePeriod(already_Add
   MutexAutoLock lock(mLock);
   mIdlePeriod = aIdlePeriod;
   return NS_OK;
 }
 
 NS_IMETHODIMP
 nsThread::IdleDispatch(already_AddRefed<nsIRunnable> aEvent)
 {
+  // Currently the only supported idle dispatch is from the same
+  // thread. To support idle dispatch from another thread we need to
+  // support waking threads that are waiting for an event queue that
+  // isn't mIdleEvents.
+  MOZ_ASSERT(PR_GetCurrentThread() == mThread);
+
   MutexAutoLock lock(mLock);
   LeakRefPtr<nsIRunnable> event(Move(aEvent));
 
   if (NS_WARN_IF(!event)) {
     return NS_ERROR_INVALID_ARG;
   }
 
   if (mEventsAreDoomed) {