Bug 1832470 - Revert to legacy boolean attribute behavior, treating disabled="false" as enabled. r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D177752

toolkit/themes/linux/global/menu.css

@@ -32,8 +32,8 @@ menucaption {
   background-color: initial;
 }
 
-menu:where([_moz-menuactive]:not([disabled])),
-menuitem:where([_moz-menuactive]:not([disabled])) {
+menu:where([_moz-menuactive]:not([disabled="true"])),
+menuitem:where([_moz-menuactive]:not([disabled="true"])) {
   color: -moz-menuhovertext;
   background-color: -moz-menuhover;
 }