Do you still have access to your current email box so you can click a confirmation link?

You can add a new "secondary" email to your Firefox Account

Then after you verify that new address, you can visit your account settings with a special link and switch the secondary to primary

See: Change the primary email address on your Mozilla account

Does that work?

Try these steps:

Open the Options/Preferences page (about:preferences). Go to the Sync tab and sign in if necessary. Click the "Manage account" button. That should open the Firefox Account settings page.

• https://accounts.firefox.com/settings

On this page there is a "Secondary email" item. Click the Add button to add a secondary email address. Follow the instructions in your verification email to confirm your second email address. Refresh the page and the "Make primary" button will now show up under Secondary email.