My favorite keyboard shortcut is command + w

That article about remapping shortcuts in OS X is nice. Sadly, it was written a while ago, and, while the method in it does allow me to add a new shortcut in Snow Leopard[1], it apparently does not deactivate the old one. Thus, my newly created shortcut quits Firefox; but so does command-Q. :-(

[1] In the instructions in the article, replace "Keyboard & Mouse" with "Keyboard". Otherwise, follow the instructions exactly.

If you're using Chrome, sometimes you can Command Shift T and it'll reopen the entire browser along with all the tabs the way it was before you shut it down.