I also avoid FF because of the UI. It's what I use on my PC, which doesn't get used much. If I have to VNC into a linux machine at work, I'll use it there as well. But that is even rarer.
The last time I noticed a UI change, it was for the worse. The made the url/navigation/search bar get bigger when it had focus. To me that is pretty dumb, and I wish you could turn it off. Knowing Firefox you probably can, but a few Google searches couldn't tell me how. If this was done on a product that already had a good UI, it wouldn't seem like a big deal. But given how much low hanging fruit there is in Firefox's UI, it makes you question their priorities.
This is all really weird, since what made Firefox take off in the first place was its better UI. A lot of websites had compatibility issues, and people wanted to use Firefox in spite of them. A lot of webdevs got to work fixing these issues because they wanted to be able to switch to Firefox themselves. It seems that Mozilla never really understood how much its UI advantage helped Firefox get popular. Instead they seem to attribute that period of success to things that I think most people consider nice to haves: extensions, privacy, about:config, etc.