the page was a strict xhtml. the design was tableless. IE was showing my left menu with all available hieght by the parent. Firefox was not. poor FF i’d say. (if it was IE, i would have cursed it. cheating huh?). The solution found after a simple googly. FF is inheriting the available height. so you would have to give 100% from the top to the bottom of the element tree.
html, body, #mainsextion, #leftmenu {height: 100%;}
