var time = 0;
var t1 = new Date ();
for (var k = 0; k < 500; k++) {
    for (var i = 0; i < 100; i++) {
    }
}
var t2 = new Date ();
alert (t2.getTime () - t1.getTime ());

Firefox: about 47ms
IE: about 31ms

Ho, there are a lot needed to be improved for Firefox!

If running some big application, the efficiency will be sharply different!