You are correct with naming convention (jscript_menu_com.js) and you have them placed correctly (src='/zencart/includes/templates/brown/jscript/menu_com.js').
Have you adjusted this line in the jscript_exmplmenu_vars.js to reflect the path to the images?
cause it appears its not finding the images on the zencart sideCode:var Arrws=['tri.gif',5,10,'tridown.gif',10,5,'trileft.gif',5,10]; // Arrow source, width and height


very much broken in FF, less so in IE. The html side works fine in IE, but as kobra said some of the links render tranparent in FF. Its gotta be some sort of javascript problem with FF. I have had problems with some scripts crashing FF, don't know why. I'm no javascript programmer.

