In no specific order: the new Proton UI, Pocket, overemphasizing Mozilla's VPN, innocuously-sounding by sometimes privacy-invasive 'studies', automatic updates, the archane method of disabling automatic updates despite having a built-in configuration UI, having to restart the browser in the middle of my workflow due to automatic updates I did not want in the first place. In general, I get the feeling that Mozilla has adopted Reddit's business model of trashing their product by contuously fixing what isn't broken.
>>nulbyt+(OP)
Thanks. Oh, I misunderstood you ;-) and thought you were talking about Brave. I stopped using Firefox quite a while ago and settled on Brave. Some config is needed but it works (very) well for me.
>>chappi+W44
Brave is okay. I don't like that copying a URL on mobile requires one tap, while everywhere else you tap and hold to select. I also hate the tiles that replaced the list of "tabs." And I don't like the fact that it's based on Chromium.