--- a/toolkit/content/widgets/popup.xml
+++ b/toolkit/content/widgets/popup.xml
@@ -717,17 +717,17 @@
         this._clearScrollTimer();
 
         // If the user released the mouse before the popup opens, we will
         // still be capturing, so check that the button is still pressed. If
         // not, release the capture and do nothing else. This also handles if
         // the dropdown was opened via the keyboard.
         if (!(event.buttons & 1)) {
           this._draggingState = this.NOT_DRAGGING;
-          menupopup.releaseCapture();
+          this.releaseCapture();
           return;
         }
 
         // If dragging outside the top or bottom edge of the popup, but within
         // the popup area horizontally, scroll the list in that direction. The
         // _draggingState flag is used to ensure that scrolling does not start
         // until the mouse has moved over the popup first, preventing scrolling
         // while over the dropdown button.