An interesting quote from ZDNet concerning its 50 million downloads mark:

Firefox late last month marked 50 million downloads of its browser since the Version 1.0 release in November. Downloads do not equal users, because a single user often downloads multiple copies for different computers or to replace the browser with patched versions, of which Firefox has had three.

Interesting. So, the more insecure it is, the more downloads will report! Hmm.... :-)

[update: a note from the comments section - autoupdates (the main way people upgrade) are NOT counted against the download figure.]