And its not a saved password? (under tools/security/saved passwords)? If it is you can delete that line.

Open the password manager. Remove the entries for that site. The next time the manager asks if you want to save the pasword, Just Say No.

Type about:preferences#security<enter> in the address bar.

Under Passwords, select Remember. To check the passwords you have listed, press Saved Passwords.

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

See also:

• https://support.mozilla.org/kb/password-manager-remember-delete-change-passwords