--- a/browser/base/content/browser.js
+++ b/browser/base/content/browser.js
@@ -8218,18 +8218,18 @@ var MousePosTracker = {
   },
 
   removeListener(listener) {
     this._listeners.delete(listener);
   },
 
   handleEvent(event) {
     var fullZoom = this._windowUtils.fullZoom;
-    this._x = event.screenX / fullZoom - window.mozInnerScreenX;
-    this._y = event.screenY / fullZoom - window.mozInnerScreenY;
+    this._x = event.clientX / fullZoom;
+    this._y = event.clientY / fullZoom;
 
     this._listeners.forEach(function(listener) {
       try {
         this._callListener(listener);
       } catch (e) {
         Cu.reportError(e);
       }
     }, this);