--- a/browser/components/customizableui/PanelWideWidgetTracker.jsm
+++ b/browser/components/customizableui/PanelWideWidgetTracker.jsm
@@ -92,19 +92,19 @@ var PanelWideWidgetTracker = {
       return;
     }
     this.adjusting = true;
     let widgetsAffected = [w for (w of gPanelPlacements) if (gWideWidgets.has(w))];
     // If we're moving the wide widgets forwards (down/to the right in the panel)
     // we want to start with the last widgets. Otherwise we move widgets over other wide
     // widgets, which might mess up their order. Likewise, if moving backwards we should start with
     // the first widget and work our way down/right from there.
-    let compareFn = aMoveForwards ? (function(a, b) a < b) : (function(a, b) a > b)
-    widgetsAffected.sort(function(a, b) compareFn(gPanelPlacements.indexOf(a),
-                                                  gPanelPlacements.indexOf(b)));
+    let compareFn = aMoveForwards ? ((a, b) => a < b) : ((a, b) => a > b);
+    widgetsAffected.sort((a, b) => compareFn(gPanelPlacements.indexOf(a),
+                                             gPanelPlacements.indexOf(b)));
     for (let widget of widgetsAffected) {
       this.adjustPosition(widget, aMoveForwards);
     }
     this.adjusting = false;
   },
   // This function is called whenever an item gets moved in the menu panel. It
   // adjusts the position of widgets within the panel to prevent "gaps" between
   // wide widgets that could be filled up with single column widgets