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:
in /home/knoppix/.config/lxsession/LXDE/autostartCode:@pulseaudio -D
fixed the playback sound problem.