Hi, do you mean using the steps in the following article:

Change primary email address on Mozilla account

If you cannot do the first step of adding an email address as a secondary address (so that you can then swap them), is it possible you already have a second Mozilla Account under that email address?

I do not think I have an account under that additional email address. I do not want to lose all my data, or I would just close the account and start over.

Is it possible that you added this email address in the past to a Mozilla Account? What happens if you try to login using that email address?

Are there other devices connected to this account? Are you losing access to the current primary email or do you want to this email for other purposes?

You will only lose data on the Sync server, you should keep data in your current profile, but you should disconnect Sync and backup your data before you try to remove the account or otherwise make changes.

Keep in mind that Sync isn't designed to be used as a backup, but merely is meant to sync data between connected devices. Best is always to backup your personal data locally.

• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles