Here's a helpful shortcut tip from tips4mac for all you
Firefox users. The next time you go to type in a URL in the address bar, stop typing before the ".com" part and hit "command-enter." This shortcut key sequence will instantly insert the ".com" part and direct you to a particular site. Try with geeksugar and hit "command+enter," it will instantly add the .com! "Shift +enter" will also add ".net" and "shift+command+enter" will add ".org."
This is a great tip, but I've also found that hitting enter will sometimes add the ".com" as well so I guess you just never know (although for the .net and .org this is really useful).