Re-post Rated 5 out of 5 stars

Got your mail Klemens Schölhorn, And checked this add-on again and found that I am using youtube html5 addon. So when I go to youtube site it gets html5 player automatically, for that reason it did not work for me. Sorry I should have checked it thoroughly before posting comment. It works for me now.
#(Previous post- Does not work on Firefox 23.1 tested on youtube.)

This review is for a previous version of the add-on (0.9.3). 

Could you please be more specific? What does not work? Have you reload the page after enabling flash (there is a setting to do this automatically, too)? You can contact me via email if you want.
It works for me (and thousands of other users) on every firefox version since 19 on windows and linux. I just tested it again on firefox 23.0.1 and it worked well on youtube.

/edit: Tanks for retesting and updating your review!