diff --git a/CHANGELOG b/CHANGELOG
index 76b8216..505f8f0 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,5 +1,7 @@
 *SVN*
 
+* Make sure Event and Event.extend are defined before wrapping events and calling their handler. Prevents a known Firefox bug from throwing errors on page load/unload (cf.: https://bugzilla.mozilla.org/show_bug.cgi?id=361271). Closes #5393, #9421. [staaky, John Resig, sam, Tobie Langel]
+
 * Minor cosmetic changes to the display of unit tests in terminal. [Tobie Langel]
 
 * Make submitting forms work in Opera < 9.1. Closes #9917, #9463, #8260. [kangax]
diff --git a/src/event.js b/src/event.js
index b516b98..b011bea 100644
--- a/src/event.js
+++ b/src/event.js
@@ -152,8 +152,9 @@ Object.extend(Event, (function() {
     if (c.pluck("handler").include(handler)) return false;
     
     var wrapper = function(event) {
-      if (event.eventName && event.eventName != eventName)
-        return false;
+      if (!Event || !Event.extend ||
+        (event.eventName && event.eventName != eventName))
+          return false;
       
       Event.extend(event);
       handler.call(element, event)