Hi, I checked with Firefox 25.0 and it is working. Which firefox version you have installed? Can you try again by disabling your Firefox add-ons? You can do that by going to Help->Restart With Add-ons disabled feature.
I think that instead of using that add-on which blocks Flash you can just turn off the Flash add-on in your browser and it will have the same effect.
Not sure what kind of changes the add-on makes besides blocking flash content. Probably it will be the best to contact the add-on developer for more details. But it is not a popular plugin, so you can be sure that it will work for all your visitors.