Oh undo function, what would I do without you? 
Well, I would probably be forced to do it again – but that is beside the point.  I love being able to reopen a closed tab or window in Firefox.  It has saved my rear end many times.  This is just one of those Firefox tips you really should memorize.

What was that tab you closed about three tabs back again? Could it have been the fourth tab back, before you got your coffee? If you find yourself using the Ctrl + Shift + T keyboard combination to bring up lost tabs a lot, there is a better way to get the job done.  The Undo Closed Tabs Button add-on for Firefox gives you a few ways to save your lost tabs.
This extension allows you to undo closed tabs [...]