I think this post's title says it all, but a word of explanation nonetheless.
One of my favorite features of Firefox is that it remembers usernames for forms. Anytime you type a username in a form Firefox will store it, and the next time you go to that form and select the username field, Firefox will display all the usernames it remembers for that particular form in a dropdown just below the field. It's pretty sweet.
The only problem is that sometimes the field gets populated with usernames you don't want in there. For instance, I've been doing a lot of work on a web application, and it involves logging in as various test users. But these users are only temporary, and I don't want them stored. Or, worse, have you ever mistakenly entered your password in the username field? I've always wondered if there were a way to clear these unwanted or temporary usernames, and it turns out there is.
A quick Google search was all it took, honestly. A Netlobo article contained the answer. But the short story is, to clear an unwanted username from the remembered username dropdown, simply highlight the username in the list and hit Shift-Delete.
And there you have it. Problem solved.