Firefox

I would give it two weeks. You stare at this interface a lot throughout the day. When it changes, your brain's immediate reaction is "That's different and different is bad", even though you may not actually dislike it in the long-term. It's certainly been growing on me.

You can change individual aspects with a userChrome.css file. For example, I might use that to reduce the tab height, as I rarely click tabs. But yeah, I wouldn't recommend trying to change everything with that. It's going to break sooner or later, as Mozilla obviously can't promise to not change the structure of their UI.