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.
Given those early reasons for favoring it, from my perspective it's mostly gotten worse since 2.0 came out.
It was nearly invisible for quite a long period.
But then I installed the Firefox iOS app, which must surely have an UI written from scratch, and it's just as weird. I'm starting to think that they just don't put enough effort into UX, or perhaps don't hire the right people.
To access many things like history and bookmarks, you need to use the dropdown menu on the right, which has like 30 different options in it, some of which seem similar but are actually different. Those options are sometimes also available from different locations, so it can be confusing how to access what you’re looking for.
Other browsers are a lot more polished in this area, like Brave, Chrome, and even the new MS Edge. None are perfect, but I find their UX and UK slightly easier to use and generally less clunky. If FF fixed that, I’d be completely sold on it. But as is, I keep going back to the drawing board.