New
#20
Benchmarks show that 64-bit version of Firefox run faster: Firefox 32 bit (x86) vs. Firefox 64 bit (x64)
Also, it could be potentially safer to run a 64-bit browser if hacks assume 32-bit data registers.
Speed and safety are good reasons I think to use 64-bit.
As for why Flash is not 64-bit, I see no reason why it can't be. It's not as if Flash content (video and games), need to strictly run within 32-bit space. Heck, even 64-bit versions of Java exists and that's magnitudes way more complex than Flash.