Youtube video cannot be played on firefox
I am NOT saying this will resolve your issue, however you can try manually wiping them to see if it works in cmd prompt.
del "C:\Windows\Prefetch\flashpl*.pf" /q /f del "C:\Windows\Prefetch\PLUGIN-CONTAINER*.pf" /q /f del "C:\Windows\Prefetch\FIREFOX.EXE*.pf" /q /f del "C:\Windows\Prefetch\FLASHPLAYERUPDATESERVICE*.pf" /q /f
You can also read about my other fixes.. clink on my profile and there is a link to my blog that explains it in detail.