From Lifehacker:

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.

The story comes from a comment:

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.

A clever trick, and a much better way than the constant www’s and .com’s. For more awesome (and often Firefox-only) shortcuts, check out the whole list of Firefox shortcuts.