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Some Linux users install FF via apt-get or something similar...so they don't count.
Some Windoze users install FF via local unofficial mirrors...so they also don't count.
Some more Windoze users install FF via bonus utility CDs coming w/ their favorite PC magazines...so they also don't count.
Some users download FF multiple times in SAME IP ADDRESS, so their download count might not be counted. (hopely so.)
Of course, there're some users out there who downloaded FF multiple times in SEVERAL DIFFERENT BOXES.
So, while number of downloads is not exactly same as number of actual users...they at least give people some idea of usage base of FF.
Warning!! Minor Trolling Ahead!!!!
I don't like watching opera of any sorts.