These would have to be my favorite worst FF extensions :

11.
Spyware Accelerator - Speeds spyware download and installation
8.
JS Crash - Prevents any page with Javascript from running by crashing the browser
5.
RandomTab - Moves tab order around randomly
4.
KidProtector - Replaces every single image with a picture of a bunny wearing a top hat
1.
Redirector - Any link you click is redirected to a random MySpace profile.

Now, the REAL evil would be #11 set to dowload #5, and #2 (sorry, you’re gonna have to click the link to find out what #2 is. . .I can’t totally steal this post.)

Read the full list here
http://www.bbspot.com/News/2006/03/top-11-worst-firefox-extensions.html