Linky
http://gemal.dk/mozilla/linky.html?ver=2.3.0
I use this to highlight all links on a page (e.g. thumbnails on a gallery) and select "open all links in tabs" or single window. It's great for browsing galleries and Text TGPs.
Search Status
http://www.quirk.co.za/searchstatus/
I use this for Google PR, Google tools (show page in G cache, show Google,MSN,Yahoo backlinks for this page), Archive.org tools (show this page in archive), Alexa tools (traffic ranking)
View Cookies
http://www.bitstorm.org/extensions/
I use this to see what cookies the current domain has set with contents
Web Developer
http://www.chrispederick.com/work/firefox/webdeveloper/
Fuck loads of tools in this one, excellent.