Structural tags are XUL tags that affect layout but that aren't necessarily visible themselves. In the simplest case, they have no content of their own.
As stated earlier, all XUL tags, including unknown tags, have the style display -moz-box applied to them. Those tags with special purposes are then overridden, leaving the rest to act like <box>. That means <xyzzy> and <happy-sad> are XUL box tags, although not particularly obvious ones. There are three tags put forward as the standard way to express boxes:
<box>. This tag is a horizontal box with defaults as described under “Common Box Layout Attributes.”
<hbox>. A horizontal box. This tag is exactly the same as <box>. The name merely helps to remind the reader ...