This interesting article is not so neutral for sure (it’s hosted on the mozilla site), but from a first reading seems very well made and supported by numbers.
The article start with “Is IE9 a modern browser? NO”
IE9 is definitely better than IE8 and a step in the right direction, but I don’t believe it to be a truly modern browser, and let me tell you why.
The article continue presenting some test done in some professional site, like caniuse and html5tests and in general all the test presented show a big difference between Firefox 4 and Internet Explorer 9.
But i suggest you to check the result presented here.