See https://support.mozilla.org/en-US/kb/add-secondary-email-address-firefox-accounts Btw Firefox Accounts has been renamed as Mozilla Accounts since early Nov 2023 to reflect that it is not being used for only Firefox Sync any more. https://support.mozilla.org/en-US/kb/access-mozilla-services-firefox-account#w_what-services-can-i-use-with-a-mozilla-account