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.