Yeah, every website has different stats about user-agents, depends a lot on the types of users you attract. I bet HN has Firefox usage ratio above 15% for sure, while sites like Instagram probably has way below the global average.
Global browser marketshare never made much sense. You need to figure out what your users use, then aim to be compatible for most of those, and ignore any global stats.
As of December 7th, Firefox is 10.3% of traffic there. This makes it the #2 browser, after Chrome (at 62.2%) -- a spot it has been dueling with Edge over for the last year or so.
I'm inclined to think that "people visiting wikipedia" is a fairly representative sample of "normal users", overall.