Having a div follow the user as they scroll - xhtml/js

I’m trying to figure out how to make this work with xhtml strict, but I haven’t been able to figure out how to set the “top” property of the div in firefox (I can get it to work with IE), and with both firefox and ie I can’t seem to retrieve a value like document.body.scrollTop or window.pageYOffset (so I can use it to move the div).

http://pjrp.unleashed.ws/testjs.shtml

Any ideas?