Need FB Comments Box to scroll *under* my position:fixed global navigation bar

I have a position:fixed global navigation bar at the top of the web page.

Currently the FB Comments Box passes over the navigation bar when I scroll down.

I want the FB Comments Box to pass under the navigation bar when I scroll down.

HTML

<div id="header">
<ul id="main-nav">
<li><a href="home.html"> Home</a></li>
<li><a href="two.html">  2   </a></li>
<li><a href="three.html">3   </a></li>
<li><a href="four.html"> 4   </a></li>
<li><a href="five.html"> 5   </a></li>
<li><a href="six.html">  6   </a></li>
<li><a href="seven.html">7   </a&开发者_高级运维gt;</li>
<li><a href="eight.html">8   </a></li>
<li><a href="nine.html"> 9   </a></li>
</ul>
</div>

<div id="fb-root"></div><script src="http://connect.facebook.net/en_US/all.js#xfbml=1"></script><fb:comments href="http://www.example.com/" num_posts="2" width="500" colorscheme="light" style="a"></fb:comments>`

CSS

div#header { width: 100%; border-top:0;  background:url(../images/tab.gif); padding-bottom:0em; **position:fixed;** }

ul#main-nav    {font-family: Arial, Helvetica, sans-serif;margin:0;padding:0;float:left;width:100%; font-size:1.1em; font-weight:normal;}

Simon

Insert style="z-index:1;"

<div id="fb-root"></div><script src="http://connect.facebook.net/en_US/all.js#xfbml=1"></script><fb:comments href="http://www.example.com/" num_posts="2" width="500" colorscheme="light" **style="z-index:1;"**></fb:comments>

Insert z-index=2 into #header