I'm seemingly mistaken :D.
[rocketmouse@archlinux tmp]$ asp update
[rocketmouse@archlinux tmp]$ asp checkout firefox
[rocketmouse@archlinux tmp]$ cd firefox/
[rocketmouse@archlinux firefox]$ grep "Features" -A2 trunk/PKGBUILD
# Features
ac_add_options --enable-alsa
ac_add_options --enable-jack
If I first launch qjackctl and then firefox (without apulse),
then firefox doesn't show up in qjackctl's audio tab. As a shot in the
dark I added the option --jack and --jackd to firefox, but nothing
changed. IIRC apps could connect to jack without being visible in
qjackctl's audio tab, IIRC I noticed this when using audacity, but I
might be mistaken.
This is firefox on Arch Linux.
[rocketmouse@archlinux firefox]$ pacman -Q firefox
firefox 56.0.2-1