Hello. Since switching to win10, Mozilla plays some movies on FB badly, plays on Youtube without problems. Other browsers on FB have no problem playing everything. I think I've tried everything. With flash plugs or with the H264 codec. I'm out of ideas. Maybe someone met with something like this?
Maybe try to disable hardware acceleration and / or all active plugins. I found on the net that someone helped uninstall Flash Player, Firefox, remove all other files associated with it and clean reinstallation; first Mozilla, then Flash.