🐛 Fix issue where MozPopupElement constant was being overwritten

Author: kierandrewett

components/panels/content/browser-panel.js

@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-const MozPopupElement = MozElements.MozElementMixin(XULPopupElement);
+var MozPopupElement = MozElements.MozElementMixin(XULPopupElement);
 
 class BrowserPanel extends MozPopupElement {
 	#debugVisiblePref = "dot.panels.debug_information.visible";