Why arn't youtube videos showing video, just audio

Just as the question says. Using the windows 8.1 version of firefox, when i load youtube videos it just shows the video box was just black, while audio plays as if normal. I tried switching from the HTML5 player to the flash player via Youtube Flash Video Player and it didn't work, the video turned green instead, but audio was just fine. i tried to download windows media feature pack, but it wouldn't work at all. if anyone can possibly help me solve this issue i'd appreciate it