The problem I have with this though is if enough people on Firefox spoof their user agent to Chrome, it's gonna look like less and less people are using Firefox and Chrome will eventually have a monopoly.
They already have a monopoly. The amount of people using FF is pretty small unfortunately. And there's a bunch of sites that only test in Chrome and sometimes even actively "block" Firefox like here without making an effort to check for capabilities instead of user agent.
I think you can spoof per tab/container. i used an exclusive Firefox Profile for the bad/contaminated (read: not privacy respecting) browsing - in there i'd ocasionally switch the UserAgent to make Teams calls.
there's no way i'd work on a machine with M$ spyware installed and always running