Solved Firefox horrible sound with vp9 codecs

I don't know after upgrade from 10.2 to 10.3-RELEASE videos with vp9 codecs (or the videos which are marked with ADP in Videodownloadhelper addon) gives nearly only noise. With deep and slow voices, like played with too less speed.
I have tried with option alsa, pulseaudio both with and without option Integer-samples on or off. Nothing changed. Any hint or suggestion?
This videos worked in all videoplayers ( smplayer, mpv-player,...) and with midori not with firefox.
Seems I was too fast. As I open this thread the last try was not ready.
Solved. It was the option integer-samples on. I turned it erroneous on, cause of an error message last www/firefox update (audio/soundtouch option)..

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Somebody else might encounter the same issue and, although it was self-inflicted, there's a good solution here.