I built firefox with jack support on gentoo linux. Firefox jack support is not useable. Firefox tries to manage its own connections, and I couldn't make it stop from doing so. I maintain my own jack connection manager.

On Thu, Aug 8, 2019 at 6:12 PM Ralf Mardorf <ralf.mardorf@alice-dsl.net> wrote:
On Thu, 8 Aug 2019 17:55:18 +0900, crocket wrote:
>mpv doesn't cause other programs to induce pops. mpv is more robust
>against pops than web browsers. ALSA loopback is inherently
>unreliable. I suspect a high quality USB soundcard could potentially
>solve the issue.

I can't comment on your set-up, however, Firefox has got jack support.

Some distros might package Firefox with pulseaudio support only, if so
you could rebuild it with

ac_add_options --enable-alsa
ac_add_options --enable-jack
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