I cannot logon to Mozilla Add-ons even using the old email address: it tells me that my old password is now too short and must first be reset. But the registered email address (and ISP) no longer exist to which password-reset authorisation would be set. Therefore I certainly cannot change either the email address or the password.

I have created a new account, which is what I am using to ask this question; but I cannot use my preferred user-name as "it is already in use". How can I delete the old account to be able to start again from scratch?