newer FF does not support alsa, got a be PulseAudio. out there somewhere in git land someone has put a stop to that but you got to run their script, and then maintain a separate update of FF. I'd suspect, because if you let your repo update FF it will over write any changes you've made to it.
ALSA support is dropped from Firefox 52 onward.
(I've seen a post in here on that alsa script for FF in here, but do not ask me where it'd be now)
what version is you FF btw?
edit: this MIGHT be it
PulseAudio emulation for ALSA
https://github.com/i-rinat/apulse
if PulseAudio is in fact installed, well then, Huston we have a problem...
But I'd rather know what version FF you are running so me and others can know how to proceed in this matter.