Resolved, kind of. Signed back into account, looked at security tab and saw the multiple entries saying account sign in failed. Refreshed 2FA, same thing. Disabled 2FA. Deleted off 2FA app. Re-enabled 2FA, creating QR code, backup codes, etc. Went to add sync again, same procedure: User name, password, email auth code but then it asked for the 2FA auth code. And successfully completed. Now have access to passwords and bookmarks that I use across various devices. Ideal Solution: Ask for 2FA up front if account exists, redirect to verify 2FA prior to starting sync process. As mozilla / ff are able to see that I am logging into an account, why not ask for 2FA when adding new device? Perhaps refresh / disable / enable allowed account to be 'updated' with 2FA usage despite having used it in the past?