I tried to display one CSS file for FF and a modified CSS file for IE. After inserting this code, I see no CSS at all for FF. IE works fine.
<!--[if !IE]>
<link href="masterFF.css" rel="stylesheet" type="text/css" />
<![endif]-->
<!--[if lte IE 6]>
<link href="masterIE6.css" rel="stylesheet" type="text/css" />
<![endif]-->