HELP with youtube video

Alright guys, I need help.

For some reason I can't view youtube videos in fullscreen.

I can see and hear the video in normal small video screen size, but when I click the fullscreen button on the bottom right it does expand but doesn't show video, just a black screen. However I can still hear sound.

I have re-downloaded the latest version of flash, but didn't help.
I did just recently updated to the latest version of firefox, and I think that had something to do with it.
I also had to do a system restore, so maybe that also has something to do with it too.

Any suggestion to try would be great, thanks.

 

Try using the latest version of Chrome. Also what's your OS? Windows? Mac? Linux? (and what version)