I need to know how to delete certain entries which autofill when this webpage's indicated field in the screendump is clicked. I want to trim the autofill list down.

I want to retain the autofill function itself.

I read the help guide does the instructions have no results.

If they are Address autofill entries, you can manage them on the Settings page. More info: Automatically fill in your address on web forms -- "Edit your saved addresses" section. (Maybe there is also a way to edit them in the drop-down, but I never do it that way myself.)

If they are Form autofill entries (Form History entries), you should be able to highlight one of them using the arrow keys on the keyboard and remove it by pressing either Delete or Shift+Delete. More info: Control whether Firefox automatically fills in forms.