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25 million downloads doesn't mean 25 million users. I've personally downloaded it at least 10x and after each test I've stop using it. Of all the people I've downloaded it and showed it to, not one wanted to use it. That counts as at least two dozen more attempts. So count me as about 30 downloads with 0 sustained usage.
And since Firefox is also available for Linux and Mac OS, what percentage of the total downloads have been for those two platforms?
Nothing beats skewing statistics huh? 25 million downloads does not mean 25 million IE switchers on Windows.