http://www.xulplanet.com/reference/elemref/ref_separator.html Creates a small separating gap between elements. This type of separator should be used to separate parts of a dialog. It doesn't draw a bar visually, but is just a small space. It is functionally equivalent to a spacer except it defaults to a small size (usually 1.5 ems). Used to specify whether the separator is a horizontal or vertical separator. Note that the values are the reverse of what seems more likely. The separator separates vertically placed elements. The separator separates horizontally placed elements. A grooved separator. A thin grooved separator. A thinner separator which is usually 0.5 ems.