Clearing form field histories in Firefox

Hello, thanks for reading...

  How do I delete a single item of form history data?

  I'm using Firefox 1.0.7 on WinXP, and I let it save my form data.
Sometimes I make a spelling error, though, which gets saved.

Over time, when I go back to that website and enter my username,
the drop down list with the form field history has both the
misspelled and the correctly spelled usernames, i.e:

  ______________
|f            ^|
|fo            |
|foxrockd      |
|foxrocks      |
|______________|


So I'd like to delete the wrong ones!  I'm wondering,
how do I delete specific items of form history data?

Thanks you know,
Matt