Did you try to delete the account?

• https://support.mozilla.org/en-US/kb/how-do-i-delete-my-firefox-account

You can possibly first try to set this old email address as a secondary email in the old account and then remove that secondary email, you need a temporary email address to use this as primary email in the old account.

You can add a secondary email address and confirm this email address. Once this email address has been confirmed, you can make the secondary email address your primary email address.

• https://support.mozilla.org/en-US/kb/change-primary-email-address-firefox-accounts
• https://support.mozilla.org/en-US/kb/add-secondary-email-address-firefox-accounts

Well it does not help still can't reuse the email from the old support account.

Marked as solved still is not!

Did someone close this?

Is the old account deleted ? You may have to wait 24 hours or longer to let this propagate to all mirror servers.