Great addon Rated 5 out of 5 stars

(Positive: Gets you to the html5 webm version, if there is one. Negative: Breaks the back button.)

UPDATE: Version 0.6 is awesome! Back button works. HD option (opt-in) is great.

This review is for a previous version of the add-on (0.2.1-signed). 

The back button problem is finally fixed! :)
The new version has not been reviewed by Mozilla yet. If you don't want to wait, you can download the new version from this site: https://addons.mozilla.org/en-US/firefox/addon/youtube-html5-video/versions/