Rated 5 out of 5 stars

make sure flashdisable is not enabled before you uninstall this add-on, otherwise flash will be disabled every time firefox starts.

This review is for a previous version of the add-on (0.8.4). 

This is a problem with the Addon Manager. I uploaded a new version which uses an other API that should fix the problem: https://addons.mozilla.org/de/firefox/addon/flashdisable/versions/
You may have to restart firefox several times (5 in my case) and each time enable Shockwave Flash using the Addon Manager to finally re-enable it.