O_o It all works ! Rated 5 out of 5 stars

I was looking for one that actually worked and you made it :D
Only thing is it doesn't seem to do it automatically as mentionned (in linux at least) but I have experienced the permanent and really annoying changes that Youtube makes every 2 months, so I understand that it won't work most times.

Anyway congratulations on your awesomeness and thank you for sharing.
Btw now I have to get used to having ZERO flash on the internet but it'll be ok, it's a slowly dying evil anyway :3

EDIT: Pffff this is ridiculous... I was notified by the dev within the minute about the existance of version 1.0.3, I upgraded and now everything works automatically with all the sizes etc... simply a must have add-on.

This review is for a previous version of the add-on (1.0.1.1-signed). 

Did you use version 1.0.3 (not yet reviewed, see description)? There I fixed the autoplay which should work on linux as well. ;-)