Please explain the problem in detail. What happens? What is the exact error messages?

Type about:preferences#security<enter> in the address bar. Look under Logins,

Note: For Firefox 56+ its; about:preferences#privacy Look under Forms & Passwords +++++++++++++++++++++++++++++++ Turn on Remember Logins. To check the passwords you have listed, press Saved Logins.

See also:

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

You can use these steps to remove saved items from an autocomplete 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 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

Note that hovering with the mouse no longer works.