I am a longtime Firefox user. I absolutely love many innovative feature Firefox has implemented (such as container tabs). Firefox does so many things better than other browsers, such as allowing CTRL-clicking tabular data for copy-and-paste.
However, I’m usually annoyed by features they add that seem like they’re just doing it to be like the dominant browser.
The worst was when they reassigned CTRL+I from getting page info to match IE’s behavior of viewing favorites. Thankfully, they’ve gone back to the sane behavior.
Actually a decent amount of people were pissed at Quantum, process per tab, discontinuation of XUL, and the new extension system. There is like a whole project to restore the ancient Firefox, and it's slow as fuck (it's called Palemoon)