It works as expected on most tabs. When the source gets updated (like after a while when watching a stream, or reloading the page), it stops working, and while that's fine I found another problem. For twitch I use a special player, because it stutters a lot otherwise, and it opens up as a moz-extension site. It straight up doesn't work in those types of sites.

Works some of the time. I mainly use it for youtube. When youtube refreshes the streaming video (after a few hours) it randomly resets so you have to set it again to your output you want. If you change your personal output it resets eventually and you will have to set it again to the output you want.

Sadly in latest 116.0.2 it didn't show up.
media.setsinkid.enabled is true now by default; maybe another Firefox function is overriding this addon?
Also not working in latest Canary versions 117.x
Tested as shown on youtube and also twitch

I haven't been able to figure out how to actually change the audio device once this is installed. The description says it allows me to do it, but doesn't say how. The similar audiooff add-on says to right-click on a window and select the additional menu item, and that one works for me.