These would have to be my favorite worst FF extensions :

Spyware Accelerator – Speeds spyware download and installation
JS Crash – Prevents any page with Javascript from running by crashing the browser
RandomTab – Moves tab order around randomly
KidProtector – Replaces every single image with a picture of a bunny wearing a top hat
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