Rated 4 out of 5 stars

As this addon breaks the Youtube's "large player", I resolved this by installing "Stop Tube HTML5" [1] and enabling the HTML5 player in Youtube. [2]

[1] https://addons.mozilla.org/en-US/firefox/addon/stop-tube/
[2] http://www.youtube.com/html5

This review is for a previous version of the add-on (1.5.15.1).