--- a/toolkit/content/browser-content.js
+++ b/toolkit/content/browser-content.js
@@ -1763,18 +1763,18 @@ let DateTimePickerListener = {
 DateTimePickerListener.init();
 
 /*
  * Telemetry probe to track the amount of scrolling a user does up and down the root frame
  * of a page, and also the maximum distance a user scrolls down the root frame of a page.
  * This doesn't include scrolling sub frames, but does include scrolling from JavaScript.
  * See bug 1312881 and bug 1297867 for more details.
  */
-addEventListener("DOMWindowCreated", function(aEvent) {
-  if (aEvent.target !== content.document ||
+addEventListener("DOMWindowCreated", function() {
+  if (event.target !== content.document ||
       content.location == "" ||
       content.location.protocol === "about:") {
     return;
   }
 
   let amountHistogram = Services.telemetry.getHistogramById("TOTAL_SCROLL_Y");
   let maxHistogram = Services.telemetry.getHistogramById("PAGE_MAX_SCROLL_Y");