Okay, in this case the problem stems from that width:auto; line in browser.css, because that permits the element to collapse so long as it won't affect word-wrapping (i.e. long paragraphs keep it stuck out)
However, you can fix the width of #leftcontent as a percentage, so that it scales with #container. Thus, the following solution appears to work in Firefox:
#leftcontent {
width: 100%;
-moz-box-sizing: border-box;
box-sizing: border-box;
}
(Those last two lines force Firefox and CSS3-implementing browsers to use the IE box model)
As well, IE may benefit if you add the following line to the top of each page (above the doctype):
<?xml version="1.0" encoding="utf-8"?><!-- This comment is meant for IE7 -->
That should force quirks mode so IE uses its old box model. (Unless you want to switch back to HTML 4.x Transitional. Also I note that your page is technically not valid XHTML as it has the wrong MIME-type, and thus should be using the Transitional DTD.)