At times we type passwords along with the user name by mistake. In Firefox, these entries are stored and whenever we type our username, the wrong entry also appears.
We usually have to clear the browser data as a whole to delete this particular mistake. To avoid doing so, we can type starting letters of the wrong entry, and the Firefox displays (drop down) all the entries starting with those letters. Just point to the unwanted entry and press SHIFT+DEL. That’s it, our headache is over… :)