Here're two more that I have:
ShowIP: Show the IP address of the current page in the status bar. It also allows querying custom services by IP (right mouse button) and Hostname (left mouse button), like whois, netcraft. Additionally you can copy the IP address to the clipboard. This extension was formerly known as ipv6ident.
Can't remember why I wanted this one but I thought it was interesting at the time.
NoScript: Extra protection for your Firefox: NoScript allows JavaScript, Java and other executable content only for trusted domains of your choice...
Pros - Eliminate flash adds (they irratate the hell out of me!) and can speed up page download.
Cons: Has to be tweeked for each page you visit that uses flash or java scripts. This includes java scripts that enable links to another page, e.g. you click a link or "next" link and nothing happens.
Then for the heck of it:
Leet Key: Transforms typed or static text to L337, ROT13, BASE64, HEX, URL, BIN, DES Morse code, DVORAK keyboard layout and to lower/to upper case functionality, Leet Font.