Currently Virtual Earth doesn't detect the version of Firefox properly.
This hack will get things working
// If the browser is Firefox get the version number
var ffv = 0;
var ffn = "Firefox/"
var ffp = navigator.userAgent.indexOf(ffn);
if (ffp != -1) ffv = parseFloat(navigator.userAgent.substring(ffp + ffn.length));
// If we're using Firefox 1.5 or above override the Virtual Earth drawing functions to use SVG
if (ffv >= 1.5) {
Msn.Drawing.Graphic.CreateGraphic=function(f,b) { return new Msn.Drawing.SVGGraphic(f,b) }
}
// Put your own code below this line
map = new VEMap('map');