This is documented here: http://www.w3schools.com/js/js_browser.asp
As an alternative, the php object $_SERVER['HTTP_USER_AGENT'] will give you the name and version of the browser in a text string, as in:
$browsername = $_SERVER['HTTP_USER_AGENT'];
This is documented here: http://www.w3schools.com/php/func_misc_get_browser.asp.
I sincerely hope that this helps, but basically you are on your own with finding a work-around for IE6. That is, unless someone here still codes for IE6?