fix for #3286

src/event.js

@@ -479,8 +479,11 @@ var withinElement = function( event ) {
 	var parent = event.relatedTarget;
 	// Traverse up the tree
 	while ( parent && parent != this ) {
+		// Firefox sometimes assigns relatedTarget a XUL element
+		// which we cannot access the parentNode property of
 		try { parent = parent.parentNode; }
-		catch(e) { parent = this; }
+		// assuming we've left the element since we most likely mousedover a xul element
+		catch(e) { break; }
 	}
 
 	if ( parent != this ) {