I recommend this extension Rated 5 out of 5 stars

For the users who have a problem to display this extension in Youtube , its works for me , and this is what i have do to solve this small issue :
1- I have just add a another extension his name is : YouTube Flash Player or others .... , his aim is to change the player of youtube video from Html5 to Flash player , and this is it when you will refresh your browser in youtube it will appear below the video . :)