This is fantastic! The amount of customisation possible with just a few seconds to spare is phenomenal. Maxime seems to get the usability point of view bang on ! :D Kudos, to the developer.
The default themes could use a bit of work to make the UI a bit less jarring, but this is just nitpicking. Nothing a couple of seconds in a user-style override can't fix.

Overall, top notch addon to youtube on firefox !I say it this way, because you'd stop using youtube without this installed once you start. This is *that* good. :)