After updating Firefox this morning to 58, it fails to play any sounds (eg. YouTube plays without sound or Google Calendar events sounds). With the as of yesterday most recent version (57.0.4?), everything worked fine. I now get the message
To play audio, you may need to install the required PulseAudio software. [Learn how]
The link there isn't of any help, as it just suggests to make sure that PulseAudio (which is required from version 52) is correctly installed. As I said, it worked fine with version 57, and I can play sounds with PulseAudio like this:
I'm running Ubuntu 17.10 (GNU/Linux 4.13.0-31-generic x86_64) with xmonad, no flash installed and the only things that updated since yesterday were Firefox and keybase.