This is one of those things that's obvious once you know it, but solves a real problem beforehand. You're using firefox with autocomplete switched on. You make a mistake filling out a form once, and find that forever onwards the mistake exists in your autocomplete list. Let's say it's a phone number, and you mistyped it. You could very easily select the wrong autocomplete entry.
Now this is cool. Select the autocomplete entry with the up/down arrows on your keyboard. Press the "Delete" key. The erroneous autocomplete entry is gone forever.
So simple, but so handy. Not something you would necessarily find without being prompted. My thanks to the tester with the mostest, Brad, for showing me that one.