Biggest dig I have on Firefox is that Dark Reader runs pretty poorly on it compared to Chrome, particularly on complex Google-owned websites like Gmail, and I can't go back to white-background-by-default anymore.
It'd also be nice if Apple actually allowed third party browsers as first-class citizens, because not having content-blocking support in iOS Firefox makes it a non-starter.
Otherwise, Quantum-based Firefox is objectively a better browser IMO. I like the extension store better, and in general it seems to work more smoothly for me. If my company weren't stuck on Google Apps where Chrome's blurring of login is actually useful, I'd use Firefox all the time. As it is, I use it as my "personal login" browser everywhere other than my phone and tablet.
(ObDisclaimer: I used to work for Mozilla half a decade ago, but I definitely would not have praised the pre-Quantum version the same way. It had fallen way behind the curve.)