Firefox 3 - Removing individual auto complete entry

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When typing into textfields, there are some values you dont want displayed again (ie. text values, random jibberish, etc)

Theres a button in the options dialog to clear everything, but I hate letting it learn all my usernames again.

Instead, only delete the ones you dont want by:
1. Start typing into a textbox and some suggestions will appear.
2. Highlight them by pressing "up/down" on the keyboard and then press the "delete" button.
3. Whalla! Gone! Wonder in amazement at how easy it was.