Lame. Personally I see this as a sort of laziness on the part of the programmers. You also see this with plugins quite often too. The development of 64 bit versions lags behind that of the 32 bit ones. I think they don't like writing the 64-bit versions of platforms because these are harder to put together and write than their corresponding 32 bit counterparts. As long as it is possible to run a 32 bit version of the browser on a 64-bit operating system though I suppose I'll be okay with it but I'm sick of all the excuse making people dole at instead of being honest and just saying they don't want to go that extra mile because it's hard and they don't like doing it.
Waterfox is better than the nightly 64-bit builds anyway
Honestly though, the 64-bit firefox really hasn't made much difference. Firefox rendering a page a few thousands of a second faster is not really worth the incompatibility with plugins. I mean, I just run both at the same time when I need to, but it's annoying.
Switching to Linux is NOT the best solution for users who are using 64-bit versions of Windows. There is no reason for anyone who want to switch between 32-bit and 64-bit OSes (including Windows). Windows 7 and 8 both include 32-bit and 64-bit licenses.