On Arch based distros Firefox is compiled with JACK support and if JACK is started Firefox will use JACK by default.
If you want Firefox to use ALSA type
about:config
into Firefox's search bar. It warns you this can bork Firefox. Proceed anyway. Now add an entry
media.cubeb.backend
and set the type to 'string'. Now give it the value
alsa
Firefox will now always use ALSA, even if JACK is started.
When Firefox got JACK support I thought that was great. In practice ... not so good. Firefox won't play back with a buffer lower than 512 without xruns. This is no use for playing along to Youtube videos while using Guitarix. 512 is too much latency. Now the latest version of Firefox doesn't auto-connect the Firefox JACK client which could be another reason people would want to use a different audio backend.
If someone would like to try forcing Firefox to use Pulse with this method, please do and report back.
Changing Firefox Audio Backend To ALSA
Moderators: MattKingUSA, khz