rori,
Делайте так
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html><head>
*!*
<!--[if lt IE 8]>
<script type="text/javascript">
location.href = 'http://www.mozilla.org/en-US/products/download.html?product=firefox-12.0&os=win&lang=en-US';
</script>
<![endif]-->
*/!*
</head><body>
В ИЕ6, 7 будет переадресация.
</body></html>