Firefox does seem to be untested for a lot of websites.

You can't do video calls with Teams in firefox for a really clear and obvious example, but I encounter breakages quite often and have to switch back to Chrome.

Safari suffers the same, but I get the impression people are testing safari more than Chrome.. but I hear so many complaints on hackernews about safari, I suspect at some point Chrome penetration will finally dislodge Safari on MacOS... or if MacOS market share goes down: then it's over and we have a total monopoly.