This is another new feature of Firefox 2.0 is the ability to restore closed tabs. In earlier versions it was available as an extension but now, similarly to Session Restore, it is part of the main functionality. If you close a tab my mistake, you don’t have to dig all over the net (or your History) to find it. Instead, right-click on one of the other tabs and from the context menu select Undo Close Tab. This will restore the tab you have closed by mistake.