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I was running the 64 bit version of Minefield, but it doesn't work with Flash, so I slowly stopped using it. What's the point in using a 64 bit browser if you have to load up a 32 bit browser frequently? The small bit of time you may save is negated in switching between the two.
It's not that it doesn't work with Flash - it's that Adobe isn't interested in coding a 64bit version of Flash.
There is currently no point or need to running a 64bit browser as this point in time. 64bit PCs have ONLY just begun becoming "popular" or "mainstream" give it some time.