I am currently on TW 10.01 snapshot. I am using gnome 41 and Firefox 92.0.1

I have three sound devices (a monitor speaker, a headset, and a laptop speaker)
I have set the output device to the monitor speaker (via the gnome's settings
pannel). However, Firefox doesn't use the device. Instead, it only delivers
sound through the laptop's speaker or the headset. Flatpak firefox and firefox
downloaded from firefox.com *do* have the same problem. 

Gnome web and other apps don't have the problem .

A workaround I've found is to plug in my headset and plug it out. Then firefox
will deliver sound through the monitor's speaker for the current pages but it
won't do that for new pages. Or if I choose a different device (laptop speaker
or headset) and then switch back to the monitor speaker, firefox will not use
the monitor speaker again.

I am not sure whether this is an upstream bug.