I'm not so worried.

I don't visit any US government sites ever, and very few corporate sites. The ones that I have problems with (Microsoft O365 in particular!!) are easily faked by setting the user agent to Edge on Windows. That magically solves everything (proving that Microsoft is deliberately breaking the experience on Firefox!).

But "Supporting" a browser doesn't really mean very much. As long as the browser is standards compliant it will still work.