I havent been keeping on top of the add ons lately but this one caught my eye. If you have loads of tabs open it can somethimes be difficult to tell whats what as they get reduced and stored in that tab thing next to the + sign on the right of the tab bar.
This extension
Firefox Showcase - Download
give you a preview pane of thumnails when you click the extra tab button next to the + sign. Loads easier to see which tab is which