Where is the drop down menu that you are referring to? Is it where you log onto your email account?
If it is then it is a browser feature that stores your log on details for any web sites that you have visited.
In Firefox if you click on where you log on it will open a drop down box. At the foot of the box it will show "View Saved Logins". Click on that and a new page will open. Select the log on details that you don't want and click on "Remove".
