both nighly and stable ff is broken - my mother couldn't access her meeting in Firefox the other day because Teams said "you only have to use Edge or Chrome"

This is absolutely true. There are some types of meetings (likely the ones which are MS Stream-based -- but as a plain user this is impossible to detect) where upon joining you will get the dreaded message about incompatible browser.

I don't know what is "support" for them, but I've never been able to make calls in it. Only the chat function works.

That's what I was thinking too. I have to keep Chrome installed on my Linux machine exclusively to use Teams for voice and video calls, since calls have never been possible in Firefox and the Linux desktop client is so buggy.

Linux desktop client seems to be dead; as of last week, trying to download the deb or rpm package results into 404.