Tab Mix Plus is a Firefox extension that can be described as the king of tab-tweaking extensions. It adds about sixteen thousand (give or take) tab-related options to Firefox, many of which you'll find in other extensions, but Tab Mix Plus is the only way to get them all in one place. It adds essential features like duplicate tab (and duplicate tab in new window), undo close tab (or window), crash recovery, single window mode, and tweaks for just about every tab behavior Firefox has, plus some handy keyboard shortcuts. Check out the features guide(PDF) to see if Tab Mix Plus has your pet feature, or head over to Mozilla Update to grab the extension.