XUL content in a given window can be covered by additional content from the same document using these tags:
<tooltip> <menupopup> <popup> <popupgroup> <popupset>
Not all of these tags are recommended, however. <menupopup>, for example, should generally be used only inside a <menu> or <menulist> tag. The following XML attributes can sometimes be applied to other XUL tags:
tooltiptext grippytooltiptext popup menu context contextmenu contentcontextmenu
Not all these attributes are recommended either. The <menupopup> tag is discussed in Chapter 7, Forms and Menus. The other bits and pieces are discussed here.
The <deck> tag can also be used for visual effects. It is discussed in Chapter 2, XUL Layout.