I use:
Adblock Plus
IE Tab
Screen grab (lets you take a screenshot of an entire web page at once)
DownloadHelper (INCREDIBLY useful- lets you save most sorts of media [streaming audio, youtube, embedded video, etc.] with one click)
ColorZilla (mostly useless, but it gives you the exact color of anything in the browser)
And I have DownThemAll and the Imageshack toolbar installed but usually disabled.
What I really need is something that will automatically open mailto: links in Gmail. I used to have one that did that, but it was never updated to even FF2.