You can try clearing the unwanted logins here:

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

Does that work?

A detail like a website remembering you (log you in automatically) is stored in a cookie.

You can use these steps to make a website remember you:

• Create a cookie 'allow' exception to keep such cookies, especially in case of secure websites and when cookies expire when you close Firefox.
• Tools > Options > Privacy > "Use custom settings for history" > Cookies: Exceptions

You can clear the cookie of a website to make the site forget about you. You can inspect and manage the cookies for the domain in the currently selected tab:

• Click the "Site Identity Button" (globe/padlock) on the location/address bar
• Click "More Information" to open "Tools > Page Info" with the Security tab selected
• Click "View Cookies" to view the cookies for the domain in the currently selected tab

• Firefox/Tools > Options > Privacy > "Use custom settings for history" > Cookies: "Show Cookies"

You can use these steps to remove saved (form) 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