Firefox won't switch audio outputs (even with a browser restart)

Problem: I try to listen to sound in Firefox through my headset, but the sound only plays through my speakers. Context: My speakers are connected using my 3.5mm audio jack, while the headset connect through a USB to 3.5mm adapter. Things I've checked: - It is not site-specific; - My system audio output is set to the headset; - System volume mixer says Firefox has sound for headset; - Audio plays through headset in other applications including Steam, Spotify, and Chrome; - Unplugging speakers from audio jack just reverts sound to laptop speakers - not to headset; - Restarting Firefox has no effect (not that I should have to restart my browser to switch between audio output devices)