menu caving in

If you visit my website sentient.mmgtv.org and view it using IE6, I get this horrible glitch where the menu caves in as if it didn’t have a fixed width.

Please view my site in the newest firefox and then view it in IE6 using browserlab.adobe.com.

What can I do to fix this? Here is the css it uses:

#menu { background:#3B3B3B; height:40px; width:960px; padding-top:5px; margin-top:10px; border-bottom:1px solid #999; }