This is a by web standards of measuring time ancient menu. Looking at the timestamps on the files (which if an inaccurate yardstick, could only be later than the actual release date) and the code I'd guess at the latest mid 2004. But it doesn't directly address IE 6 which came out in 2001. So either it came out earlier, or the author saw no reason to acknowledge IE 6 in the code. The fact that it has served you so well all this time is a testament to how well it was written, and perhaps says something about how old your site design is.
I mention that not to cast aspersions upon your site (I haven't looked at it) - more to say it might be time for a complete overhaul/update, including a more modern menu.
Anyways the example.htm from the distribution archive doesn't work for me in Firefox 4 (if I added some content to the page it did). So, if as you say things work for you in Firefox and IE up to but not including IE 9, then you could opt to use compatibility mode for your pages - forcing IE 9 to act like IE 8 (requires a meta tag). But before you do that try editing browser.js in the includes folder. Find this area and add the highlighted:
Code:
else if (document.all)
{
var version = parseFloat(agent.substr(agent.indexOf("msie")+5, 3));
if (agent.indexOf("mac") != -1 && version >= 5)
ie5_mac = true;
else
{
if (version >= 9){
dom = true;
}
else if (version >= 5.5)
ie55 = true;
else if (version >= 4)
ie4 = true;
}
}
worked for me in IE 9 for the example.htm mentioned above.
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