I found this while using another addon of mine, and I destroyed darkmatters, facebook.com, and several other places with it. Cause it's an addon, it won't be deleted from your system.(like NetDisaster)
DOWNLOAD FROM HERE
Then to start the game you find this icon located in either your AddressBar or your status bar and click. it will( if this is you first time playing) ask if you want to read the rules, you should read them. Then if you on a page like www.destroytheweb.net, that has frames, the inner frames will be removed before the start of the game.
I played for 30 min and I'm keeping this addon for firefox