I have a hard time imagining how they enforce that. What keeps a malicious program from replicating the exact changes that Firefox makes when installing an extension? What about just replacing the whole profile folder with one that has a malicious extension installed?

>Firefox has absolutely mandatory code signing for extensions

That helps I guess, but there are clearly still malicious extensions that can pass the automated tests and get signed. Even if that wasn't possible, you could probably use some userscript extension and load malicious scripts that way.