In IE9 h/w acceleration and Direct Write are controlled by the document's rendering mode. Hacker News renders in quirks modes and thus uses GDI. Try forcing the document to render in IE9 mode (open the F12 developer tools, hit Alt-9) and the text will look much closer to Firefox 4's. The page layout will also break a bit.
I assume it is due to the Direct Draw, but I don't see the same issue in IE9 w/ hardware acceleration enabled.