Just curious, what has been breaking for you? What UI features have changed in some significant way since Australis?
SINCE Australis? Nothing major. In a recent version they changed the right click context menu to include icons for reload/back/forward, which irritated me - change for the sake of change. (Also the keyboard shortcut for Private Browsing no longer works - might be a plugin? Not sure.)
Things like that seem little but when you've been using Firefox for years - which I have, every day, for work - little changes like that mean the platform loses a lot of stability, which is one of the things that is most important when you're trying to get things done.
I'm not at all opposed to new features. I don't even care about feature bloat that much. But they should be opt-in. And at the very least, you should be able to opt-out without having to install some third party plugin. Having a new UI/UX forced on me just feels ... rude.
Australis prompted me to install Classic Theme Restorer so I could restore the browser to the way I'd been using it for /years/. (Here's my +5 post about why I disliked Australis.) Enough has been written about Australis so I won't whine about that any more.