http://www.xulplanet.com/tutorials/xultu/springs.html http://www.xulplanet.com/reference/elemref/ref_spacer.html An element that takes up space but does not display anything. It is usually used to place spacing within a container. If you don't specify that the spacer has a size or is flexible, the spacer does not occupy any space. If you want a small gap, consider using a separator instead.