IE9 supports ecma-262 edition 5 (http://en.wikipedia.org/wiki/ECMAScript) and Firefox 4 is javascript 1.8.5 (https://developer.mozilla.org/en/JavaScript/New_in_JavaScript/1.8.5).
How compliant is IE9 with ECMA-262 edition 5 and is that the same as ECMAScript5?
I expect there are more differences than similarities between the two javascript engines, but I am curious how close IE9 has moves toward Firefox, in terms of new javascript functionality.