About 24% of Firefox users still run a 32-bit OS, though gamers probably have newer hardware and software than the average Firefox user. See "Browsers by Architecture" in the Firefox Hardware Report for a graph over time:
As depressing as it is to say this: Firefox's marketshare is so low that we really need the Chrome numbers here. 24% of a <10% market share means "quite likely around 2.4% globally". That's basically a number that says "safe to entirely ignore in terms of userbase support" =/
