--- a/browser/base/content/tabbrowser.xml
+++ b/browser/base/content/tabbrowser.xml
@@ -4393,17 +4393,17 @@
               if (PrivateBrowsingUtils.isWindowPrivate(window) !=
                   PrivateBrowsingUtils.isWindowPrivate(sourceNode.ownerDocument.defaultView))
                 return dt.effectAllowed = "none";
 
               if (window.gMultiProcessBrowser !=
                   sourceNode.ownerDocument.defaultView.gMultiProcessBrowser)
                 return dt.effectAllowed = "none";
 
-              let copyModifier = this.AppConstants.platform == "macosx" ? event.altKey : event.ctrlKey;
+              let copyModifier = this.tabbrowser.AppConstants.platform == "macosx" ? event.altKey : event.ctrlKey;
               return dt.effectAllowed = copyModifier ? "copy" : "move";
             }
           }
 
           if (browserDragAndDrop.canDropLink(event)) {
             // Here we need to do this manually
             return dt.effectAllowed = dt.dropEffect = "link";
           }