There is more information on how to remove specific passwords from Firefox in the Viewing and deleting passwords section of the Password Manager - Remember, delete and edit logins and passwords in Firefox documentation.

You may also need to follow the instructions in the Deleting individual form entries section of the Control whether Firefox automatically fills in forms documentation.

Hope this helps.

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.