Still running Knoppix 7.7.1 but wanted to test that so-called 'bug' of FF.

I reloaded Synaptic, installed pulseaudio, pluseaudio-utils 10.0.-1 and pavucontrol 3.0.3-1. then upgraded Firefox to version 56.0-2 (not a recommended action).

The entry made by pulseaudio in /etc/xdg/lxsession/LXDE/autostart does not seem to be enough for pulseaudio to start roperly.

Adding the line:

@pulseaudio -D
in /home/knoppix/.config/lxsession/LXDE/autostart

fixed the playback sound problem.