Create an empty menuseparator in a FireFox extension

Here is a quick tip on how to add an empty separator into a FireFox extension menu. By default, the XUL menuseparator tag draws a line across the menu. Here is how to add a separator that just adds a space (no line):

<menuseparator style="visibility:hidden" />

Pretty simple, but it took me a little searching to figure out, so I thought I would post it here.