Is that a good idea though, considering that your browser is arguably the largest attack surface there is on your computer?

It's not a good idea. That's also why it's very important to provide updates that do not disable functionality important for the user. That way you don't force users to choose between security or functionality.

Mozilla still does not allow non-store extensions on their new firefox. Until they're back I have to keep old Firefox around. ...which sucks.

Me too. Waiting for the current branch to properly support extensions, or for an Android Chrome fork to support the extensions I use.

Kiwi Browser let install Chrome extensions :)

Yes! I've tried that, but one of my most-used save page extensions doesn't work.