Just by peeking into your markup with IE Dev Toolbar, first suggestion I could give is to give yourself a wrapper-div to hold all of your elements.
<div id="myContainer">
<div class="header">...</div>
<div class="menu">...</div>
...
</div>
You can then apply rules to the outer-most container, "myContainer".
#myContainer {
width:800px;
margin:auto;
}
For example. This would keep your site a fixed width, and center it up in the browser. The reason I mention the width is because in FF it looked similar to IE, until I expanded the width of my FF browser...then everything fell apart.
Another thing I would suggest is getting rid of your "leftSpacer" and "rightSpacer" divs. Instead of using that method, use left-margin and right-margin on your leftCont and rightCont.
Might consider this markup, or something similar:
<div id="container">
<div id="header">
<div id="mainNav">...</div>
</div>
<div id="content">
<div id="leftCont">...</div>
<div id="centCont">...</div>
<div id="rightCont">...</div>
</div>
<div id="footer">...</div>
</div>
Just a couple suggestions. I'll take another look and see if anything else immediately strikes me.
Of relevancy, check out
www.SampsonVideos.com if you're just getting started with CSS etc
