My doctor's weird video chat doesn't work in Firefox (and even in Chrome it's barely functional probably because it hasn't been updated since before the pandemic), but other than that singular example, everything else works fine. I think most people parroting complaints about Firefox just haven't used it recently enough to realize that it's fine in 99.9% of cases.

It's rarer than is used to be but I know Ive had issues in the past with firefox on some insurance websites, some banking websites, I know Ive seen sites where they block you from even entering but if you change the user agent it works. It's less common than it was 5 years ago though as there is a report website feature and they work hard to try and fix compatibility.

It wasnt firefox's fault though more the web developer. I dont have specific examples because I didnt save them and it's been a while. Its not as bad as during the ie6 days of the internet though. Now that was dark times.

Edit: Also sometimes it's addons and people mistake that for the browser and dont tweak their addons before blaming firefox.