Teams was a classic hellhole for me when I was using Linux. Nowadays it lets you in but Firefox genuinely doesn't support all of the features the Teams site uses. This means certain features are made unavailable to Firefox users. Forcing the user agent makes some of these partially work but they are also still broken so Mozilla doesn't want to enable a user agent override by default and Microsoft doesn't want to enable a feature that only half works. This GitHub WebCompat issue serves as a good example history https://github.com/webcompat/web-bugs/issues/77892

Haven't used Zoom in browser much so can't say what one can find there. I just run the desktop app. It's definitely something where you can go a few years with everything working fine then suddenly hit like 4 sites where you're like "wtf, why is shit so broken?". Chipotle and similar joints' online order pages were another place that would spontaneously break.

Yeah, it's pretty rare. Like now that I think about it, it isn't even that Zoom doesn't work -- it's that Zoom-in-Firefox-on-Linux doesn't seem to have the fake background thing and I need that because it's easier than cleaning my garage-office :)

And I'm guessing that's either deliberate or lazy on Zoom.