We currently have no good UI for:

a) setting this sort of a preference
b) letting the user know why they'd want to pick one or the other

Moreover, I don't think the scripts have been tested correctly for
running, say, both 32 and 64-bit browser instances at the same time (presumably,
they'd trip over the lock file.)

At this point, I think this is WONTFIX.

I was thinking command line, not starting it from menus or so, but I can see the
point.  As for the scripts, if you have a running firefox, running any other
firefox will just get the running one to open the requested URL, due to
communitation (I suppose) through the X server.  So, yes, it's confusing.

But if we could just add it to the x86_64 distro, even if not installed by
default, it would make things much simpler on the update side.