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" =/
other way around: it assumes most non-firefox users are on 64 bit, which is a pretty reasonable assumption given Apple's hard push for 64 bit (so that's the OSX-ers covered) and Chrome's "we've made finding the 32 bit installer _really hard_ because we don't want you to use it" move. I'm sure there are some 32 bit users in both segments, but far less than you'll find in firefox land.