Bug 1391401 Disable legacy extensions by default on Android r=kmag

MozReview-Commit-ID: 9InGShBHQag

--HG--
extra : rebase_source : 93d5609f38d25bde7e6827b317be5f84807f79c2

mobile/android/app/mobile.js

@@ -236,6 +236,8 @@ pref("extensions.webextPermissionPrompts", true);
 pref("extensions.webextensions.base-content-security-policy", "script-src 'self' https://* moz-extension: blob: filesystem: 'unsafe-eval' 'unsafe-inline'; object-src 'self' https://* moz-extension: blob: filesystem:;");
 pref("extensions.webextensions.default-content-security-policy", "script-src 'self'; object-src 'self';");
 
+pref("extensions.legacy.enabled", false);
+
 /* block popups by default, and notify the user about blocked popups */
 pref("dom.disable_open_during_load", true);
 pref("privacy.popups.showBrowserMessage", true);