Context

Since FF 52, the support for ALSA has been officially dropped.

I don’t know about the details of the Linux audio subsystem, and I don’t have special requirements for it, other than it can play sound from multiple sources. So I just installed ALSA, and use dmix for the latter.

Void Linux provides the [firefox package][] compiled with support for alsa, sndio and pulseaudio, the latter being the blessed option which is officially supported by the FF team.

After setting security.sandbox.content.syscall_whitelist to 16 in about:config, audio worked.¹

Problem

After upgrading to version 59.0.2 during this month, audio broke in Firefox.

Solution

Uninstall Firefox. Only half-joking at this point.

Set or create the following about:config entries²:

• security.sandbox.content.read_path_whitelist: /dev/snd/,/home/username/.asoundrc
• security.sandbox.content.write_path_whitelist: /dev/snd/

The trailing slashes are important. Don’t omit.

How it works?

Due to security concerns, Firefox is now running each tab in its own isolated sandbox environment. While that’s a good thing for a browser, ALSA requires r/w access the /dev/snd/ device, which we add to the whitelists by ourselves.