Erm… I found a cool extension 8)
Is Called Flash® Player for YouTube™ 1.0.1
It disables your HTML5 and add Flash Player! 
Works For Me! 
So… no way this will not work so !ot!
mine only uses html 5 how can i change it to flash ? since it is activated and i still have the problems but in the link in this topic is still saying that i have only html 5
i hate it that some are stuck at 360p and others are in hd but that are only the pretty old videos older than 2 years
I use the Enhancer for YouTube extension. This gives me the option of displaying YouTube videos with either HTML5 or Flash.