« XUL Reference
selectedItem
Type: element
Holds the currently selected item. If no item is currently selected, this value will be null. You can select an item by setting this value. A select event will be sent to the controlling container (i.e. the listbox, richlistbox, radiogroup, etc., not the list item that was selected) when it is changed either via this property, the selectedIndex property, or changed by the user.