orient

orient
Type: one of the values below
Used to specify whether the children of the element are oriented horizontally or vertically. The default value depends on the element. You can also use the -moz-box-orient style property.
horizontal
Child elements of the element are placed next to each other in a row in the order that they appear in the XUL source.
vertical
Child elements of the element are placed under each other in a column in the order that they appear in the XUL source.

See also