Unfortunately, my favorite add-on, “Undo Closed Tab” is not compatible with FF 3.5, but I’m sure it will be soon, or that a developer will make something similar for it.
Actually, the “Undo Closed Tab” add-on IS available for 3.5. I was wrong.
How does that add-on compare to the built-in “History->Recently Closed Tabs” (which is how I reopen a tab I accidentally closed)?