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Completely agreed that this could not be the real number. You have to take in count dynamic IP addresses, or addresses under VPN. This all leads to somewhat innacurate number, with off course people that did not find FireFox to their taste. I myself downloaded FF more than few times with each consecutive release and still can not find a reason to switch from Opera.
One other thing is that FF will come and comes with variuous magazine CD's, so this would also affect the final number of people who are actually using FF
It is good though that finally there is some interest in other browsers other than IE.