Rated 5 out of 5 stars
It needs some adjustments, mate, now that -as usual- Google made some changes. Is there any nightly build or something?This review is for a previous version of the add-on (188.8.131.52-signed).
Yes, there is: http://nightly.crpt.de/firefox/youtube-all-html5/ (But note that it will NOT update automatically to the next nightly or release)
I am already fixing the last issues and will upload the new version today.
Edit: I uploaded the new version 2.1.0 which fixes all problems with the changed youtube code.
It is not yet reviewed by mozilla, but you can install it manually here: https://addons.mozilla.org/en-US/firefox/addon/youtube-all-html5/versions/2.1.0