diff --git a/mozilla/extensions/irc/xul/content/static.js b/mozilla/extensions/irc/xul/content/static.js
index 51aa0d7f387..615df212f14 100644
--- a/mozilla/extensions/irc/xul/content/static.js
+++ b/mozilla/extensions/irc/xul/content/static.js
@@ -3340,7 +3340,7 @@ tabsDropObserver.canDrop =
 function tabdnd_candrop(aEvent, aDragSession)
 {
     if (aEvent.getPreventDefault())
-        return;
+        return false;
 
     // See comment above |var tabsDropObserver|.
     var flavourSet = this.getSupportedFlavours();