What Firefox add-ons do you use that are useful for programmers?
+1 for LORI ("life-of-request-info"). It's a very convenient alternative for rough measurements of the load time of a particular web page -- the kind of thing that you might otherwise use an external stopwatch for.
New Tab Homepage. Combined with a "speed dial"-type homepage (a personal, fast-loading page of links that you use frequently), helps you get where you're going faster when you open a new browser tab.
LastTab. Changes the behavior of Ctrl+Tab to let you navigate back and forth between your most-recently-used tabs with repeated presses of Ctrl+Tab, the same way that Alt+Tab works in Windows. Also provides a nice view of all open tabs while Ctrl is still being held down for easy navigation. (The resultant behavior is very similar to the Ctrl+Tab behavior in recent releases of Visual Studio.)