Weird, I will have to check again it could be that there was some sort of internet interruption while it was checking for updated versions for add-ons.
No when I first upgraded to Firefox 4.0 it wasn’t compatible and was automatically disabled. A few days later I searched for it and was able to install it.
The guy who wrote it must have upgraded it to work with 4.0.
I love that Flashblock add-on!