I actually ran into the same problem as you a couple days ago, where I remembered the email of an account I owned, but forgot the username.
I had to track down through my messages from where I won the giveaway to find out the username.
I remember back then you could press "forget password", or some button that gives you the Username, Password, Security Question/Answers, etc...
Otherwise, not quite sure, but you could try and search for all your crossfire messages on your email to see if you find a username.