menuitem.type

type

Type: one of the values below

Can be used to create checkable menuitems or for radio button menuitems.

checkbox

The menuitem is checked. The checked attribute determines whether the menuitem is checked or not.

radio

The menuitem becomes part of a radio group. Other menuitems that have the same value for their name attributes are part of the same radio group. Only one menuitem in the same radio group can be checked at once.

More information on adding checkmarks to menus in the XUL tutorial