Tested in FF36 with GreaseMonkey and in Opera with Violent monkey. This script handles only HTML5 videos, click-to-play of Flash videos is supported even in browsers itself. If you want to change a video player to a HTML5 one, you can do so there
. Also note that YouTube changes quite frequently, they will eventually break this.
Donation of any amount would be appreciated - PayPal
27/6/2015 - Added check for recursion (should fix freezes).
18/3/2015 - Added screenshot.
18/3/2015 - Improved description.
17/2/2015 - Initial version.