In order for ALSA sound to work on a non default audio device in Firefox 59, you need to add this to about:config security.sandbox.content.read_path_whitelist = /home/youruser/.asoundrc
Same problem
I'm not sure what mozilla team is going to be able to do about this one, I don't know of a way to handle this within the ebuild. If it were a system path, sure, but not on a per-user basis.. I think users of alsa-only with non-default audio settings will just need to adjust their about:config on their own.
jack audio also fails on Firefox 59 unless one set: security.sandbox.content.level 0 Using the security.sandbox.content.read_path_whitelist = /home/youruser/.asoundrc setting loopback interfaces etc work with security.sandbox.content.level 2 (the default level is 3). ttps://bugzilla.mozilla.org/show_bug.cgi?id=783733
Problem persists with Firefox 60
It works in Firefox 60, I messed up before. This can be closed. You just need to upgrade to Firefox 60 and no about:config option is necessary.