The center you used is not W3C compliant and FF renders it correctly, in fact you are using a bug in IE to get it to center.

If you add a div to your skin

<div id="centerall">

the rest of the skin

</div>

Then add

#centerall{
   width:600px;
   margin-left:auto;
   margin-right:auto;
}

Your skin should be centered.

Leave the code you had before in for IE, since it doesn't interpret the above CSS correctly

Another way is to use a table (have a look at the CSS of this website)