I usually don't remember keyboard shortcuts, and I rarely use them. But this is one Firefox shortcut I made myself remember, because it turned out to be really useful. It's Ctrl+Shift+T, which opens the last tab you've closed. If you press it several times, it opens several last tabs you've closed, in the reverse order. It's basically an undo function for Firefox.
When you surf as much as I do, you often close tabs, just to remember that you forgot to check out the URL, or some detail, like a name or a number on the page you just closed. No problem - Ctrl+Shift+T, and it's back.