Package firefox-official-xx (62.0.3) sound was not working. Requires pulseaudio.
I installed pulseaudio and pavucontrol and following the instructions of @mojo
http://forum.slitaz.org/topic/issue-with-pulseaudio#post-37164
http://forum.slitaz.org/topic/firefox-520-audio-requires-pulseaudio#post-44307
Now pulseaudio works.
Now firefox-official-xx updated to version 63.0.1 and audio working Ok!