There are two types of suggestions in drop-downs below form fields. One type is a login (username and password pair, linked to a specific site), which should be listed on the page opened with menu button > Passwords The other type is form history. These options tend to look very basic compared with saved logins. To remove a form history suggestion, use the down arrow key to select it on the list and then press Delete or Shift+Delete to remove it. More info on form history: [[Control whether Firefox automatically fills in forms]].