Javascript runs in all the latest versions of Opera, IE, Chrome and Safari (for windows)

I have been working on the website stsontario.org a while ago for someone. I have used a javascript file to apply a background element with a specified height based on the height of the document after loading the page. It shows on the other four browsers but does not show on Mozilla. Both my javascripts are external files imported near the end of the body tag. In order to debug the code I made an attempt to put document.write("hi"); at the top of my file and even found this does not show on the page. Why does my javascript code not work on Mozilla! :'(