The only site I have compatibility issues with on desktop is MS Teams and even then it's only for voice/video calls, everything else works fine.
Firefox Android is a slightly less happy place. The password manager doesn't work very well (am moving away from the built-in one) and I can't log in on Amazon (which is important because I can't buy Kindle books in the app because of the Play Store).
This is why I don't use FF (although I'm on Linux). It's unusably slow for me. My experience is not the most common one (indicating that there's something about my ecosystem that FF hates), but I haven't been able to make FF work in any of the releases starting a couple of years back, I think.
I don't browse on my phone at all, so I won't be using FF there purely for that reason.
I don't doubt your experience, but it's clearly not universal.