Pretty much as soon as they added multi processor support they had to start throttling background tabs anyway, same as chrome. It's not at all clear that going multi process was a win. And years after they change their are still a bunch of things extensions did that don't work anymore. I was never a huge fan of XUL or any non-native UI, but it did it's job for years and it's replacement doesn't.

And due to their follow google nature of monthly updates there's no stable version to fall back on.