Rated 5 out of 5 stars

This is EXACTLY what I was looking for! One cannot always escape Adobe Flash Player but one should always try because Flash is truly crap. With due respect to John (see below), that is the behavior that I was looking for: I WANT web sites to switch to HTML5 whenever possible and they generally will not if Flash is activated. So, I installed this add-on, deactivated Flash, and now enjoy the videos on the New York Times web site in HTML5. When I must use Flash, I click the FlashDisable icon on the toolbar (and immediately click it off again when finished with the page). This saves having to go to the add-ons menu, select plug-ins, and activate/deactivate Flash every time. (A screen refresh may be necessary after toggling, however). Great add-on. Thank you very much, Klemens!

EDIT: I should add something. I have seen people review similar add-ons, saying "Why do you need this? Firefox now allows setting Flash to 'Ask to activate' and even remembers the setting for each web site." True, but that still does not accomplish what I was trying to do. I want Flash to be OFF unless absolutely necessary. When it is deactivated, some web sites default to HTML 5 or Silverlight automatically. That is what I want. Without this add-on, if Flash is set to "Ask to activate," the NY Times web site merely prompts you to activate it and I still have to turn Flash off altogether to get the site to play HTML5. Well, now I can do the trick with one click of the FlashDisable icon (and a screen refresh, perhaps). Bravo!

This review is for a previous version of the add-on (0.9.3.1-signed). 

Thanks for your extensive review and apt description! :-)