--- a/browser/base/content/browser-gestureSupport.js
+++ b/browser/base/content/browser-gestureSupport.js
@@ -721,19 +721,17 @@ var gHistorySwipeAnimation = {
   _completeFadeOut: function HSA__completeFadeOut(aEvent) {
     gHistorySwipeAnimation._removeBoxes();
   },
 
   /**
    * Adds the boxes that contain the arrows used during the swipe animation.
    */
   _addBoxes: function HSA__addBoxes() {
-    let browserStack =
-      document.getAnonymousElementByAttribute(gBrowser.getPanel(),
-                                              "class", "browserStack");
+    let browserStack = gBrowser.getPanel().querySelector(".browserStack");
     this._container = this._createElement("historySwipeAnimationContainer",
                                           "stack");
     browserStack.appendChild(this._container);
 
     this._prevBox = this._createElement("historySwipeAnimationPreviousArrow",
                                         "box");
     this._prevBox.collapsed = true;
     this._prevBox.style.opacity = 0;