Rated 3 out of 5 stars

It was a bit tricky to figure out... It seems to be incompatible with the FlashBlock Addon.
Bye and thanks

This review is for a previous version of the add-on (2.71). 

To work around the FlashBlock issue, please allow Flash on YouTube. Just right click on the Movie while it's blocked and select "Allow Flash from this site" from the context menu.