Oh boy, I have tons of these: Tamper data, Undo closed tabs button, Flagfox, GooglePreview, IDM extension...lol...They're literally countless. But I don't use toolbars, though. I hate them. Ads and redirection and stuff. Also incompatibility with different browsers at many occasions.
Measure it to check pixels Colorzilla to pick colors from websites Copy All Urls Copies and pastes all urls of open tabs inclusive history in structured form to and from the clipboard. Copy as HTML Link Creates an HTML link to the current page using the selected text and copies it StumbleUpon Toolbar Tab Mix Plus Tab browsing with an added boost. Especially Tab mix plus has some advantages when dealing with tabs.
Colorzilla is great for identifying colors on websites. Screengrab is great too for when I need to save a webpage.
I use rank checker, no follow, twitter toolbar and contribute firefox addon. I got rid of all the ones I don't use as my browser was getting cluttered. LOL
Of course Firebug and Web Developer Toolbar, furthermore Greasemonkey extension, that has a lot of tweak for several websites.
I always use 'WOT' Add-ON with FireFox. It tells you which site is reliable and which is not. It is really a good one, you can simply give it a try.