I don't know why this broke in the first place, all I know is that I had no sound and that the device selection menu was borked where other video calling webapps worked just fine. The enumerateDevices API has been in Firefox for years [1].
To be fair(ish) to Microsoft, I did spoof Chrome's user agent because Microsoft forced a "this website doesn't work with your browser" screen in Firefox. Perhaps the failing API was an unstable Chrome-only API that they assumed works because of the UA. The browser compatibility screen communicates that they definitely tested it, that they definitely knew about the problem, and that they just didn't want to fix it.
I just dropped Teams as an option and sent everyone who wanted to video chat Jitsi links. That worked fine while I needed it. If a company chooses not to support me, then I will choose not to use that company if I can.
[1] https://caniuse.com/?search=enumerateDevices