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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.
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/