If you are referring to the little drop-down menu that appears under the email address text box, this is because you have set Firefox to remember your form data. More information on this feature can be found in the [[Control whether Firefox automatically fills in forms]] documentation. If you are referring to Google automatically showing your account in a list of accounts remember on that device, this is done using cookies that Google stores on Firefox. You would need to either prevent Google from storing cookies or tell Firefox to delete your cookies when it closes. More information can be found in the [[Cookies - Information that websites store on your computer]] documentation. Hope this helps.