Yeah, Firefox has a feature called "code correction". Basically if someone leaves out a closing tag for some part of the page code (like a </div> or a </td>) Firefox will automatically fill it in or fix it. This has lead some developers to be lazy and just write a quarter of the code necessary for a webpage - and when people complain about poor site design they say "switch to firefox". It's lame.
This issue is not unique to Internet Explorer, but it happens in Opera too.
The issue you have is that you typoed one of the tags. Check your navbar template for this
Code:
<td id="navbar_search" class="vbmenu_control"><a href="search.php" accesskey="4" rel="nofollow">Search</a> <script type="text/javascript"> vbmenu_register("navbar_search"); </script></td<td class="vbmenu_control">
^The red text highlighted is where you messed up. Without that
</td> it thinks that your forum listing is part of the navbar which
is centered.