Exactly.

Why even have an official channel, providing visibility and official oversight, if when it comes down to it, you're just gonna push remote code updates through the same side channel a potential hacker would use?

People are saying it's for convenience. OK, but then they have to understand that doing things in that fashion is a really bad look. And now your users are set up to believe that, at least some of the updates coming from the side channel are "trust"-able.

This is a piece of code downloaded from Mozilla servers to re-enable extensions, which are other pieces of code you download from Mozilla servers. If your threat model includes not trusting Mozilla servers then you've presumably disabled browser extensions (or sideloaded them) and this issue is irrelevant to you. If you do use extensions and get updated versions from Mozilla I don't see any way in which this increases your attack surface.