Hello Preston.

Go to Tools > Options > Privacy, make sure that the dropdown menu says "Use custom settings for history", and disable "Remember search and form history". Of course, that will make Firefox forget ALL things input into any forms, not just the usernames.

If you want it to forget just the usernames, then it's possible that there's some extension out there that does it, but I'm not aware of any.

Also, you can easily erase autocomplete entries by doubleclicking any input field, a dropdown with all autocomplete entries will appear, and you can tap delete (keyboard) will hovering the entry you want to delete. Holding delete will erase them all one by one but faster. The dropdown only shows entries for the select field, so you won't be able to delete usernames on fields like "email" or "cellphone".

Disabling that setting will prevent Firefox from remembering new names, but the old names are still saved. You will have to remove names via Delete browsing, search and download history on Firefox.

Form History Control: https://addons.mozilla.org/firefox/addon/12021