See --> https://addons.mozilla.org/en-US/firefox/addon/saved-password-editor/

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Delete the currently saved entry and create a new entry.

To remove saved data from a drop down list, use these steps:

1. Click the (empty) input field on the web page to open the drop down list
2. Highlight an entry in the drop down list
3. Press the Delete key (on Mac: Shift+Delete) to remove it.

• http://kb.mozillazine.org/Deleting_autocomplete_entries