diff --git a/toolkit/content/widgets/browser.xml b/toolkit/content/widgets/browser.xml
index 11ed998cca..b039e9b1f7 100644
--- a/toolkit/content/widgets/browser.xml
+++ b/toolkit/content/widgets/browser.xml
@@ -1199,7 +1199,7 @@
               }
               case "DOMMouseScroll": {
                 this._autoScrollPopup.hidePopup();
-                event.preventDefault();
+                aEvent.preventDefault();
                 break;
               }
               case "popuphidden": {