You can sideload if you have the extension file.

The proper response, imo, is to implement third party add-on repos, so if Mozilla is forced to remove access to something, someone else can make a mirror or something. That way someone could create and host a repo that has blocked extensions and Mozilla doesn't get in trouble for it.

There should absolutely be a line drawn here. Mozilla shouldn't make any code changes to any of their services to appease censorship orgs (e.g. domain block lists). Blocking access to services that can be hosted/replaced by someone else shouldn't be an issue.