diff --git a/src/event.js b/src/event.js
index 307907aa0..e2bac6f9b 100644
--- a/src/event.js
+++ b/src/event.js
@@ -616,8 +616,9 @@ jQuery.event = {
 		beforeunload: {
 			postDispatch: function( event ) {
 
-				// Even when returnValue equals to undefined Firefox will still show alert
-				if ( event.result !== undefined ) {
+				// Support: Firefox 20+
+				// Firefox doesn't alert if the returnValue field is not set.
+				if ( event.result !== undefined && event.originalEvent ) {
 					event.originalEvent.returnValue = event.result;
 				}
 			}
diff --git a/test/data/event/triggerunload.html b/test/data/event/triggerunload.html
new file mode 100644
index 000000000..a7879c772
--- /dev/null
+++ b/test/data/event/triggerunload.html
@@ -0,0 +1,18 @@
+<!doctype html>
+<html>
+	<script src="../../jquery.js"></script>
+	<script>
+		var called = false,
+			error = false;
+
+		window.onerror = function() { error = true; };
+
+		jQuery( window ).on( "beforeunload", function( event ) {
+			called = true;
+			return "maybe";
+		}).on( "load", function( event ) {
+			$( window ).triggerHandler( "beforeunload" );
+			window.parent.iframeCallback( called && !error );
+		});
+	</script>
+</html>
diff --git a/test/unit/event.js b/test/unit/event.js
index 2a9cd22f3..0cd181252 100644
--- a/test/unit/event.js
+++ b/test/unit/event.js
@@ -2522,6 +2522,11 @@ testIframeWithCallback( "Focusing iframe element", "event/focusElem.html", funct
 	ok( isOk, "Focused an element in an iframe" );
 });
 
+testIframeWithCallback( "triggerHandler(onbeforeunload)", "event/triggerunload.html", function( isOk ) {
+	expect( 1 );
+	ok( isOk, "Triggered onbeforeunload without an error" );
+});
+
 // need PHP here to make the incepted IFRAME hang
 if ( hasPHP ) {
 	testIframeWithCallback( "jQuery.ready synchronous load with long loading subresources", "event/syncReady.html", function( isOk ) {
