Bug 1541045 - Drag space checkbox should be grey out when disabled r=Gijs

Differential Revision: https://phabricator.services.mozilla.com/D27065

--HG--
extra : moz-landing-system : lando

toolkit/content/widgets/checkbox.js

@@ -30,6 +30,7 @@ class MozCheckbox extends MozElements.BaseText {
 
   static get inheritedAttributes() {
     return {
+      ".checkbox-check": "disabled",
       ".checkbox-label": "text=label,accesskey",
       ".checkbox-icon": "src",
     };