In the last few months, I've seen a huge upsurge of sites that simply won't work for me in Firefox.
Not sure if it's the FF + Linux combination. Should probably try with a fresh profile to confirm.
It sucks, but I'm in on FF all the way. If much of the web stops working for me, it just means I have more time on my hands!
Slack "huddles" (video chat) work on everything except Firefox+Linux. Haven't tried changing my user agent yet, only just got onto Slack for work.
You can change your UA and add Sec-CH-UA to match Chromium if you want to avoid the (non-ADA compliant) browser bias.
Anecdotally my own experience is consistent with this with the same platform+browser combination.
It appears that there are now significant numbers of sites - or at least noticeable parts or features of sites - that rely on Google-specific APIs and haven't been tested on other browsers.
However visiting those sites from Apple devices is often similarly frustrating. I'm not sure this is an anti-Firefox thing. It seems more of a not realising there are other browsers apart from Chromium-based ones thing.