Take a look at this documentation page: [[Autofill logins on Firefox]]

Three other ways a user name (but not a complete login) might be populated into a form are: (A) Form history See: [[Control whether Firefox automatically fills in forms]] (B) Browser cache Reload the page bypassing the cache with one of these methods: * Shift+click the Reload button on the toolbar * Ctrl+Shift+R * Ctrl+F5 key (C) Page script using a value stored in a cookie To remove the site's cookies. while viewing a page on the site, click the lock icon at the left end of the address bar. After a moment, a "Clear Cookies and Site Data" button should appear at the bottom. Go ahead and click that. Firefox will ask you to confirm; go ahead and confirm. Then try reloading the page. Have you been able to track it down and clear it?