I'm always installing and uninstalling extensions. It's nice trying out all the new ones. Anyway, here are the ones I currently have installed:
BBcode - adds a right-click menu item with handy formatting options for forum textboxes. Recent versions also include XHTML formatting, which I use quite a bit when posting to my blog.
Sage - an RSS reader extension for Firefox. It integrates with Live Bookmarks, but it's much more feature-complete, with newspaper views, OPML support etc. I couldn't live without an RSS reader, and Sage is fantastic.
Super DragAndGo - mentioned above. This was originally an idea in MyIE2/Maxthon that got ported over as a Firefox extension. Very, very handy.
Linky - "Linky will increase your power to handle links. It will let you open or download all or selected links, image links and even web addresses found in the text in separate or different tabs or windows."
ReloadEvery - can set a page/tab to automatically reload every few seconds/minutes.
All-In-One-Gestures - just started using this a few days ago. Basically enables mouse-gestures in Firefox. Once you get a hang of it, it makes navigation incredibly fast.