This is getting interesting. For me on Windows 7 Firefox (both 7.0.1 and the current Beta version), Chrome, Safari and Opera all have this issue while IE9 does not. Which version of Firefox are you running?

Edit: Additionally, if I use the so-called "F12 Developer Tools" in IE9 to switch the browser mode to IE8 the font will have the same problem.

Edit 2: I found this article http://www.owlfolio.org/htmletc/legibility-of-embedded-web-f... - I also found a few references to being able to activate hinting for the fonts used in the embedding process. Not sure what the options are from TypeKit (the service used on this site) and I can't check on my own TypeKit account as their website is currently down for maintenance.