This happens to me all of the time. I'll be using Firefox  to surf the net, open way too many tabs, close a few to open more and realize that I need to go back to a website. Well luckily there are a couple of tricks to help you get your tabs back.
All you have to do is hit 'Command+Shift+T' for a Mac or 'Shift+Ctrl+T' for a PC and your last tab will appear. If you don't like keyboard shortcuts, just go to history and recently closed tabs and voila- you will see all your recently closed tabs!