I emptied the Firefox cache and the Error Console, then tried the map again.
Unfortunately, the overlay still does not appear, and my brief look at the Console indicates that basically the same problems that were there earlier today are still being reported, including the failure to support svg:
"Error: uncaught exception: Msn.Drawing.Exception: Your Web browser does not support SVG or VML. Some graphics features may not function properly."
I note that TedW was able to see the map correctly with an earlier version of Firefox/MAC; I'm using Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.2) Gecko/20070219 Firefox/2.0.0.2 - Build ID: 2007021917, and I rather suspect that the problem(s) lie in changes from Firefox 1.5x to 2.x. (I don't have a copy of FF Windows 1.5x to test my theory.)