Tab Mix Plus is a great extension for Firefox. One thing I do not like about a default Firefox install is the appearance and functionality of its tabs. Tab Mix Plus provides a stylish new tab button, close tab icons on each tabs and tab session saving functionality all in one small extension. Everything works smoothly and there are a tonne of configuration options for everything from keystrokes to mouse buttons. Overall probably the best extension to Firefox I have used so far, hopefully it gets folded into the main tree in the future.
David Harrison's musings on collaboration and I.T.