It appears that Firefox and Virtual Earth don't play as well together as I had thought. I've been working on implementing a map for one of our clients to show just a basic display of their location. Things were going fairly well until I noticed Firefox 2 wasn't firing the onMapLoad() callback I had set up. Using Firebug, I discovered that the following happy little error was being thrown during MapLoad():

"Msn.Drawing.Exception: Your Web browser does not support SVG or VML. Some graphics features may not function properly."

This didn't prevent the map from displaying, but it did prevent MapLoad() from completing, hence: no onMapLoad() and therefore (in my situation), no map positioning or pin layout. After digging around a little, the best I was able to come up with based on this discussion was as follows:

try {
    map.LoadMap();
}
catch (e) {
    //Browser does not support SGV or VML     
    onMapLoad();
} 

A hack at best, but for now it seems to resolve the issue . . .