[yabones]

I believe on Gentoo you can compile Firefox without pulseaudio by disabling the USE flag. I'm not sure if that forces it to use ALSA or disables audio entirely though. I do use pulse, so I've never disabled it globally/for Firefox.

https://packages.gentoo.org/packages/www-client/firefox

https://packages.gentoo.org/useflags/pulseaudio