On the SpreadFirefox Web site Monday, Mozilla developer and community coordinator Asa Dotzler thanked the "tens of thousands" of affiliate members who use "buttons, banners, and links to spread the word about Firefox."

Although the 200 million download mark doesn't translate into that number of users -- some have downloaded multiple versions, others canceled the download, still others downloaded but don't regularly use Firefox -- Mozilla's has acquired about 15 percent of the global usage share in the nearly 21 months since it was released. In the same time span, Microsoft's Internet Explorer saw its share drop from the high 90s to just under 80 percent.