I could have sworn that I already linked to the Mozilla Metrics blog but I don't see it in the archives so here it is again.
The data and analysis that I'm most excited about right now is outlined at the Firefox's Funnel Factor blog post.
Our crack team of metricists, (metricians?) Ken Kovash and Alex Polvi, have been studying the Firefox adoption process and have determined that we have an adoption rate of 28%. So, for every 100 users that successfully downloaded Firefox, 28 of them were Firefox users 30 days later.
If this rate has been constant and the fall-off after 30 days is negligible (two big ifs,) then our 445M downloads so far would yield about 125M active Firefox users.
But what about other sources besides downloads from mozilla.com? The 5-10M(?) Linux desktops that shipped with Firefox as the default browser? The downloads that come from download.com or tucows.com? Bittorrent? Other sources?