The window element is the root element of xul files. The xmlns attrinute defines the default namespace the basic tags belongs to. It must be always present. The meaning of the element menubar is obvious, the elements menu define individual menubar items. The value of attribute value appears as the name of the given item.

By default the menubar appears at the left side of the window.

<?xml-stylesheet href="css1.css" type="text/css"?>
<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
     <menubar>
          <menu value="AAA"/>
          <menu value="BBB"/>
          <menu value="CCC"/>
     </menubar>
</window>

xul2.xul

The align attribute with value vertical moves the menubar to the top

<?xml-stylesheet href="css1.css" type="text/css"?>
<window      xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
align="vertical">
     <menubar>
          <menu value="AAA"/>
          <menu value="BBB"/>
          <menu value="CCC"/>
     </menubar>
</window>

xul3.xul

In this example the stylesheet is not used.

xul4.xul

If you do not plan to modify the default view you can address the default stylesheet directly

<?xml-stylesheet href="chrome://global/skin" type="text/css"?>
<window      xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
align="vertical">
     <menubar>
          <menu value="AAA"/>
          <menu value="BBB"/>
          <menu value="CCC"/>
     </menubar>
</window>

css1.css

With CSS you can tailor final touch of the document. This instruction imports default Mozilla stylesheet. It is very reasonable always import some default stylesheet and then make only required changes.

@import url(chrome://global/skin);