It looks like if anything is typed in the log-in side of the checkout page and you decide to check out as guest it pull the log-in email and loads it into the fax line of billing info and for some reason it thinks your are trying to Register. Works fine if the box is empty or you Register.
@tapebrothers - We are aware of this problem and from our tests this is limited to FF if you choose to remember your password on a login or admin login page. So this limits the affect on online shoppers that either have an account already or are trying to checkout as guests. We are working on a patch that should be available Monday the 24th.