To be fair, Firefox is planning on extending the chrome API for addons after (or really around the same time, in all likelihood) it reaches compatibility as wel

To be fair, Firefox is planning on extending the chrome API for addons after (or really around the same time, in all likelihood) it reaches compatibility as well. If it weren't, Firefox wouldn't realistically be able to deprecate XUL addons, because there would be no alternative for many of them.