Sounds like you have something like IE Auto-Form Complete or Firefox's feature that saves your Username/Password to sites enabled.
Why didn't I think of that :-)
Can it be corrected when it happens? , It's only in FF for me
The only way I can think of to correct it is emptying the password boxes and re-typing the username, but it's easier to just prevent it from happening at all - Just go into your FF options and delete the username/password record for your site.