It seems to be an upstream bug according to this: https://bugzilla.mozilla.org/show_bug.cgi?id=1430274 (fixed in Firefox 59 ?)
In the meantime, I found and tested two workarounds:
* If you want to continue using ALSA without installing PulseAudio, you can whitelist the blocked syscall adding it to `security.sandbox.content.syscall_whitelist` in about:config (in my case `16`)
* Another way is to use PulseAudio as a simple bridge to ALSA, configuring it as described here: https://wiki.archlinux.org/index.php/PulseAudio/Examples#PulseAudio_as_a_minimal_unintrusive_dumb_pipe_to_ALSA (credits to Max-P who wrote the guide)
Maybe my sound is safe for another day...