Ubuntu 20.04 / Firefox: you have been disconnected

https://meet.jit.si in Firefox would always report “you have been disconnected”.

No video camera is being used, only a mic.

Any reason for the malfunctioning ?

It works for me using Firefox 68.9.0esr on Debian 10 (and I don’t have a camera either).

If you are using a more recent version, I would try Firefox ESR from mozilla.org (even if you have to install it in /opt).