Unless you've been to the site before, Firefox won't automatically add .com, .net or .org to the site you're typing in. Using your Control or Shift keys, or both together, you can quickly add the right domain ending to a site.
Photo by leko-m.
Commenter siggy13 points out this trick, and swc oxcart notes that it's as old as — well, it seems older than YouTube. Still, most of us probably aged beyond it:
So apparently in Firefox, if you're typing in the address bar and you hit Ctrl+Enter, it will append .com and take you to that site. Similarly, Shift+Enter appends .net and Ctrl+Shift+Enter appends .org. I found this out earlier when I accidentally hit Shift and Enter at the same time.
Chrome doesn't seem to offer the same top-level domain filling, and I was only able to get a Shift+Enter combination working in Internet Explorer 8. So it's mainly Firefox users that can save themselves the typing, if their hands get used to that keyboard combination, but it could be a long-term time saver.