I think the parent is talking about the difference in rendering between Firefox and Safari on OS X, not between Windows and OS X.
I last compared only a few days ago. When I get back home, I can try again and see if I can get more analytic about what the difference is, but, as I said, part of the problem with this kind of aesthetic/design judgement is that I don't have the vocabulary to analyze it.
The difference is more obvious in Cyrillic text on the bottom -- there's something wrong with hinting in Firefox.
And another one: http://i.imgur.com/op1bj.png