When you say that your secondary email address already exists, do you mean that it's showing up on the list currently or do you mean that you get that error message when you try to add it. If you are getting that error when you try to add that email address as a secondary address, it means there's already a Firefox Account using that email address. You will need to close the account associated with that email address before you will be able to add it as a secondary email address to your account. If the email appears on the list as a secondary email address, but you don't see the button to change it to the primary address, it's possible that you haven't completed the email verification yet. That needs to be completed before you can change that address to the primary on your account. Hope this helps.