I'm a long time Firefox user. I want to see it remain independent and useful. I don't like the fact that Chrome has come along and slurped up half the Firefox userbase as I'm inherently untrusting of Apple, Google and Microsoft. But there's no denying that it got a lot less user friendly over the last while.
The first annoying change was with version 4 and The Stupid Orange Button. TSOB meant I had to teach old dogs new tricks again, but it wasn't that hard. At least they stuck with it for a long time. The UI change in v29 or whatever was horrible however, and I still don't like it. Not enough really to get around to trying to make it look like it used to (although it could yet happen) but enough to be actively annoyed. The Pocket nonsense was annoying. The Hello nonsense was annoying. Most annoying of all however is the fact that one tab going funny kills them all. It's less common than it ever used to be due to plugin containers and stuff, but it does still happen. And it bugs the hell out of me because Chrome fixed it what feels like decades ago.
I see the whole thing as quite simple really - there's a section of people who don't want to be spied on and like openness and there's a section of people who don't want to be annoyed and of course there's some who will only ever use IE because it's already there. If Firefox can be both open and not annoying again, it should take the majority of users who actually care.