If you used this link below and it doesn't work then the account might be lost account at this point.

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

To be able to change the primary email address, you need access to the old (current) email address and since you wrote that you can't this email is your only option to create a new account.

Unfortunately, if you can't access the primary email address, your only option is to create a new Mozilla Account with an email address you can confirm.

• https://support.mozilla.org/en-US/kb/ive-lost-my-firefox-sync-account-information