pulseaudio --start --realtime
I can confirm this did indeed resolve the problem on FreeBSD 12.1-RELEASE-p7 in Firefox. Thanks!Try starting pulseaudio...
As root, add yourself to the pulse-rt and pulse-access groups; then, as your normal user, in a terminal, run
pulseaudio --start --realtime
If I'm right, that should do it. If not, go to Help > Troubleshooting in Firefox, scroll down one fold, and paste the Sound section.