Rated 5 out of 5 stars
The app I was looking for!Not only block flash, but really enable/disable it. Really cool to enjoy YouTube in HTML5 in combination with the "YouTube Flash to HTML5" addon (see Sukant Garg).This review is for a previous version of the add-on (0.9.3.1-signed.1-signed).
I have also made an addon like "YouTube Flash to HTML5": https://addons.mozilla.org/en-US/firefox/addon/youtube-all-html5/
(the new version is not yet reviewed by mozilla, so has to be installed manually)