take a look at this code
why doesn't relative positioned div expand? (in mozilla only)
Code:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html>
<head>
<style>
body{
    height:100%;
    margin:0;
    padding:0;
}

#tst1{
    position:relative;
    width:40%;
    background-color: #FFF8DC;
    height:100%;
    
}

#tst2{
    position:absolute;
    left:41%;
    top:0;
    width:40%;
    background-color: #FFFAF0;
    height:100%;  
}
</style>
</head>


<body>

<div id="tst1">
tst1
</div>
<div id="tst2">
tst2
</div>

</body>
</html>