Here is a screenshot of my Cloudfront dashboard showing the breakdown of user agents into particular browsers when an article of mine reached the top of HN about two months ago:
Looking at the number of distinct IP addresses in Athena for that day, about 20,200 of those hits were distinct viewers:
This is not definitive, but it's something I guess. Here are a few observations based on that data:
1. On the desktop, it looks like Chrome and Firefox are at about parity for HN users. I kind of doubt this actually, and think it's more likely that Chrome still beats Firefox by a sizable margin. But even if this is off, it looks like HN users actually do use Firefox significantly more than the general population, which means the evangelization here isn't just a vocal minority among HN users.
2. On desktop Safari is a distant third. It would appear more people browse HN in any mobile category than browse HN on desktop using Safari.
3. On mobile, mobile Safari dominates. Then mobile Chrome has about half as many users as mobile Safari, and mobile Firefox again half as many as mobile Chrome.
EDIT: I pulled an Athena report of the referrer breakdown as well, if anyone's curious: