--- a/xpcom/threads/nsITimer.idl
+++ b/xpcom/threads/nsITimer.idl
@@ -61,17 +61,17 @@ typedef void (*nsTimerCallbackFunc) (nsI
 
 native nsTimerCallbackFunc(nsTimerCallbackFunc);
 
 /**
  * The callback interface for timers.
  */
 interface nsITimer;
 
-[scriptable, uuid(a796816d-7d47-4348-9ab8-c7aeb3216a7d)]
+[function, scriptable, uuid(a796816d-7d47-4348-9ab8-c7aeb3216a7d)]
 interface nsITimerCallback : nsISupports
 {
   /**
    * @param aTimer the timer which has expired
    */
   void notify(in nsITimer timer);
 };