var listener = {
    handleEvent : function(aEventItem) {
      switch (aEventItem.type) {
        case 'quit':
          if (!confirm('are you OK?'))
            aEventItem.preventDefault();
          return;
        case 'unload':
          alert('OK, let's exit Firefox.');
          Application.events.removeListener('quit', this);
          Application.events.removeListener('unload', this);
          return;
      }
    }
  };
Application.events.addListener('quit', listener);
Application.events.addListener('unload', listener);