Hit the undo shortcut (Ctrl+z/Cmd+z) in Firefox's address bar to move through your recent history via keyboard and find tabs you've closed—sort of like a lightweight version of the Undo Closed Tab feature.
You can re-open closed tabs in browsers like Firefox, Chrome, and Opera by hitting the Ctrl+Shift+T shortcut (or Cmd+Shift+T on Mac), but if you want to go back further without diving into your History menu or re-opening several closed tabs, just focus the address bar (Ctrl/Cmd+l or Alt+d [Windows only]). Watch the video above for an in-action demonstration.
Update: Reader Ashwin Mudigonda points out that redo shortcuts—Ctrl+y (Windows) or Cmd+Shift+z (Mac)—move you the other direction in the history. Makes sense.
This isn't limited exclusively to closed tabs, but everything you've been browsing will be in there, and it's a nice, keyboard friendly alternative to using your History. The shortcut has a few kinks (once you hit undo, it appears that when you undo a URL, it's no longer available in subsequent tabs, but it's a nice hidden feature to add to your arsenal.