FEBE (Firefox Environment Backup Extension) allows you to quickly and easily backup your Firefox extensions. In fact, it goes beyond just backing up -- It will actually rebuild your extensions individually into installable .xpi files.
Calculator has an interface similar to Microsoft Powertoy Calculator and it offers great features like history, conversions, many functions... And it's a great open source replacement to the calculator included in Windows accessories!
CoLT (short for "Copy Link Text") makes it easy to copy either a link's text, or both the link text and its URL in a format you specify. This non-invasive extension simply appears as context menu items when you right-click a link. Fully customizable!