I feel a lot of the "complaints" about Firefox are misinformed opinions from a decade ago or more. I hear falsehoods such as "it's slower" or "it takes up more RAM" which are both patently false in modern times. That was all fixed up years ago.

Sadly, I suspect there are also a lot of culture war aspects driving some of the most vocal Firefox hatred.

(There's a steelmanned form of their argument - that Mozilla ought to be held to a higher standard than their commercial competitors - which you might think would speak in favour of LibreWolf and the like. But those who complain most forcefully about Firefox often turn out to be Chrome / Opera / Brave users)

It had a pretty annoying bug on Mac (which I ass-u-me was a memory leak) up until a couple of years ago. Every time you'd try and tab back to it, there would be a delay that would very, very slowly get longer over time until you closed and opened it again. I switched over to Arc at the time but after The Browser Company so epicly dropped the ball, I switched back to FF and it's been really great ever since.