I don't think it's necessarily that it's more difficult but that there is less demand. On linux distros (especially since most stuff is open source) they try to

I don't think it's necessarily that it's more difficult but that there is less demand. On linux distros (especially since most stuff is open source) they try to get everything be 64 bit on 64 bit installs, some distros don't even ship 32 bit system libraries by default. That's why linux users wanted 64 bit flash so much(you need 64 bit plugins for a 64 bit browser although there was a hack called nspluginwrapper to let you use 32 bit flash in 64 bit fiefox). I'm guessing there is a somewhat similar push on OS X to get all 64 bit software on a 64 bit os. On windows, firefox is just a single installed program(unlike linux where firefox's library might be shared with thunderbird, etc.), and a number of apps are 32 bit, and thats fine. Plus there is a larger selection of plugins many of which may never get a 64 bit version. Also I may be wrong but I think that browsers these days have a lot of per architecture optimizations targeting x86 and separately arm(x86-64 optimization being a lower priority), so that 64 bit browsers may actually be slower.