Yeah me too, ive been trying to fix it for several months. Its a hopeless timewaster.
Firefox developers are too busy with thier fast release cycle, adding fancy features to keep up with Google chrome, so they just leave broken things broken.
I use opera now for videos.
Have now progressed, but there remains a problem to be solved.
I uninstalles Flash. I then disabled anti-virus, firewall, spamfilter and vulnerables.
Installes flash (which it found the latest version to be the correct one to install), 11-5-502-110. When I wen tinto WATERFOX, it worked perfectly.
When i restarted the desktop later, waterfox failed to display using Flash.