Can you provide any details of browser usage between the 2 sites? My guess is HN has more Firefox (as it's open source and hacker-friendly) and proggit has more mobile.
https://docs.google.com/spreadsheet/pub?key=0AvcaBxBg441ZdFp...
Reddit actually has more Firefox. And HN has more IE. I guess more people are living in corporate hell than we like to think.
Chrome: 45.5% Firefox: 30% Mobile: 15% Safari: 5% IE: 2.8% Opera: 2%
... and one visitor allegedly on a PS3.