firefox: don't protect any domains from extensions

OK, so this simultaneously hurts security as extensions could modify the page,
but it may improve privacy by blocking analytics and it can improve
accessibility for any accessibility extensions.

etc/firefox/policies/policies.json

@@ -211,6 +211,11 @@
       "Enabled": true
     },
     "Preferences": {
+      "extensions.webextensions.restrictedDomains": {
+        "Status": "locked",
+        "Type": "string",
+        "Value": ""
+      },
       "pdfjs.enableScripting": {
         "Status": "locked",
         "Type": "boolean",