Menus and forms go together like wine and cheese. Mozilla's menu tags are built out of simpler XUL tags, just like most other XUL tags discussed so far. Menus, however, are a bit more complicated for several reasons:
Popup menus don't fit into the normal flow of a two-dimensional XML document.
Menus contain many separate values (menu items), not just one value.
Menus have complex internal structure.
Menus can be used separately from forms, but they still need to fit in with the constraints imposed by other form elements.
Mozilla does not use XUL menus for so-called listboxes, which lie flat inside an XML document. The <listbox> tag fills that role. XUL menus are only used for popup-style menus. Mozilla's menus follow the design rules ...