You can clear a saved login here:

"3-bar" menu button (or Tools menu) > Options > Security > "Saved Passwords"

Success?

If Firefox offers to save it again for that site, you can say "Never". If it saves it again automatically, we'll have to give you some additional steps to try.

Thanks very much.

You can use these steps to remove saved form an name/password data from a drop-down list:

1. click the (empty) input field on the web page to open the drop-down list
2. highlight an entry in the drop-down list with the mouse or cursor Down key
   do not click the mouse or press the Enter key
3. press the Delete key (on Mac: Shift+Delete) to delete the highlighted entry

• click the "Site Identity Button" (globe/padlock) on the location/address bar
• click the arrow to expand the security message
• click "More Information" to open "Tools > Page Info"
• click "Permissions" to inspect and modify permissions, see also about:permissions (44-)
• click "Security" to inspect and modify cookies and passwords in a standalone window