I found the following solution: http://superuser.com/questions/741355/html5-video-playback-is-choppy-and-corrupted-in-firefox
Just go to edit>preferences>advance tap and disable "Use hardware acceleration when available".
Maybe this could help in old computers and netbook.I think I should have made the tittle of this post more specifically like "Youtube html5 video choppy" or something. Hope it helps some one out there.
Thank you all :)
