What account?

Hi Edith, did you receive a notification that an unknown device or service connected to your Mozilla account (formerly called a Firefox account)? There isn't much more information available than what you see in the email. On the connected services list at https://accounts.firefox.com/settings#connected-services does the mystery connection have a globe icon, or a device icon (like a computer, phone, or tablet)? I think a globe indicates a login through a browser or other program rather than a normal sync. Try disconnecting the mystery connection and update your password.