Bug 1855151 has some example of confusion this causes, and bug 221490 has some more history. I don't see why chrome://foo/content should be different from chrome://foo/skin etc, in terms of privileges. I guess in the past this distinction probably made more sense? Per discussion in the review comments, we're not touching langpacks (yet). Differential Revision: https://phabricator.services.mozilla.com/D189232