Bug 1711719 - Disable new tab menulist if preference is locked. r=Gijs,preferences-reviewers

Differential Revision: https://phabricator.services.mozilla.com/D143610

browser/components/preferences/home.js

@@ -112,6 +112,7 @@ var gHomePane = {
       if (newValue !== menulist.value) {
         menulist.value = newValue;
       }
+      menulist.disabled = Preferences.get(this.NEWTAB_ENABLED_PREF).locked;
       // If change was triggered by installing an addon we need to update
       // the value of the menulist to be that addon.
     } else {