Firefox - Set Audio Output Device For Tabs Individually - FREE
(((Hot-To)))
Send audio from tab#1 to device#1, send audio from tab#2 to device#2, send audio from tab#3 to device#3.....
That's what this add-on can do. One can listen to something while another one is watching a video, that's what I read. Use your imagination to benefit from this add-on. :)