find the HTML error that screws up this page in IE (but not Firefox)

The website <[login to view URL]> is rendering correctly in Firefox 3, but when viewed in Internet Explorer 8 (and probably other IE versions, but I didn't check), the main center content gets pushed down underneath the left-hand column. I'm sure it's a very simple change (probably just the addition of a single tag somewhere) to fix it, but I can't find it myself. Tell me what I need to change, to make the page render correctly in IE8 the way it does in FF3 :)