I running endeavouros and i tried to get discord screenshare (specifically tabshare) to work on firefox.
No matter which distro i tried it on before that, never worked (chromium is the only one that works so far)
Searching for any related answer brought me to only one single related topic suggesting to install "ffmpeg-libs", which didnt help.
Discord has poor Linux support. The best you can do is use an audio graph tool like qpwgraph, helvum, or qjackctl paired with pipewire to route your tab's audio out through your mic. There is no app audio stream like there is on Windows discord.
and it works perfectly with chromium browsers so there must be something specifically with firefox that is missing, since like i said, it can tabstream, firefox cant