Volume from one Firefox tab immediately applies to all the rest
In addition to even worse issue of #698 (comment 799428) that leads to permanent hanging of video on Youtube and whole pipeline, I've noticed that, if that is worked around by explicit definition of rate and everything works generally smoothly, sometimes videos are being muted or have volume that's not what's rendered in player's slider.
It seems like it caused by automatic application of 0% volume on all tabs/streams if one was silenced. I use Youtube addon to force autoplay off and mute by default as fallback for the damn Youtube trash player not stopping anyway. So on every reload of page or player it gets to 0% due to it trying to autoplay again, and it mutes all the rest with it. But sometimes all streams are silenced on seek or closing a tab which is likely have the same cause: application of 0% state to everything. 0% volume is also getting written into ~/.config/pipewire/media-session.d/restore-stream sometimes.
So there is probably some kind of problem with differentiating different streams having the same name and parent app.