Since M. didn’t know about this the other day, I thought I’d mention it, in case it helps someone else: Firefox has a massively-useful feature I’ve been using all the time: reopen the last tab that was closed.
You can find it under History → Recently closed tabs. There’s a Recently closed windows list to go with it.
Reopen last tab is bound to Alt-Shift-T for me, though your mileage may vary, since I’ve adjusted Firefox’s bindings to suit my modifier keys (what’s a Meta for?).
I should also add that Alt-1 is bound to “Go to 1st tab”, Alt-2 to “Go to 2nd tab”, and so forth. I sometimes find this to be quicker than moving my hand off of the keyboard to click on a tab.