It is being worked on [1].
Firefox was a supported browser but they took it off the list in January until this issue is fixed. [2]
[1] https://github.com/jitsi/jitsi-meet/issues/4758#issuecomment...
Can confirm Firefox-dev on Linux has no working video.
iOS doesn’t support videoconferencing in the browser - you have to download the app.
Works fine though not tested mic(muted on system).
AFAIK Firefox on GNU/Linux uses Gstreamer. Maybe you need to install gstreamer codec packages.
Update: Firefox switched to FFmpeg and removed Gstreamer support some time ago. I have FFmpeg installed from rpmfusion, but I don't know if that's relevant to WebRTC and Jitsi.
(from a desktop Linux user years 1996-2004)