the problem that you will have when you disable overallocating memory is that when your 200M
firefox process tries to spawn a 2k program (to handle some mime type) it first forks, and
will need 400M of ram, even though it will immediatly exec the 2k program and never touch the
other 399.99M of ram.
with overallocation enabled this will work. with it disabled you have a strong probability of
running out of memory instead.
yes, it's more reproducable, but it's also a completely avoidable failure.