Getting message: "Primary email account required for sign in"

I hope I got your intent correct - adding a secondary email address.

You need to use the primary email account to authenticate the addition of a secondary email address for a Firefox Account. Once that is done you can use the secondary email address to log in to your Firefox Account; IOW, "change the email address for your Firefox Account".

See also:

• https://support.mozilla.org/en-US/kb/change-primary-email-address-firefox-accounts
• https://support.mozilla.org/en-US/kb/forum-response-firefox-sync-verification-email-faq

The System Details list shows you run an older Firefox version that is no longer supported with security updates.

Please update to the current Firefox 60.0.1 release.

• Firefox -> About Firefox
• https://support.mozilla.org/en-US/kb/update-firefox-latest-version