Delete Firefox auto-complete entries

You know the scenario: You've accidentally entered some wrong username or other information into a form field and now Firefox always suggests this wrong data in the grey auto-complete dropdown too.

It's easy to delete a single entry from the grey autocomplete dropdown:

Just start to type in into the form field until the grey dropdown appears, then use the cursor keys to move to the entry to be deleted and press the Shift key and the Delete key

Just for completeness: In IE (Internet Explorer) just press the "Del" key to clear the selected/highlighted string.