My experience is that Firefox usually sends output to the selected system default output device. When I want to use headphones, I have to change the system setting (using the icons down in the lower right of the Windows taskbar). If your Firefox is stuck on one device even after you change system settings, hmm, that sounds like a bug. There may be some pages that can detect and allow selection of output devices. Sites were denied access for many years due to privacy concerns, so even though it is possible now, I don't know whether any sites important to you will add that feature. There are a couple of add-ons that should work for inline audio and video players, but the way their permissions work (very few permissions required), they do not work for players embedded in a frame from a different server, which probably is the most common format you'll encounter. So it would be great if Firefox could provide a selector somewhere. Mozilla has a product suggestion site at https://connect.mozilla.org/. I think if you have time, it would be a good idea to post this request there to get into the current feedback stream. Use the "Ideas" section of the site.