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.