If you are the kind of person who opens a lot of duplicate tabs, and blank tabs, and you find it annoying to have kept them open, this addon is for you. It closes duplicate tabs (and retains just one of them), and blank tabs. Works on the latest Firefox (tested with 14).
It adds a button on the addon toolbar (the tool tip for the button says "Close blank and duplicate tabs"), which when pressed, closes duplicate and blank tabs.
It also adds the keyboard shortcut Ctrl+Shift+d (note Mac users, it is Ctrl not Cmd) to perform the same operation (d stands for delete duplicates and blank tabs).