Since launch last week in excess of 17.3 million downloads of Firefox 3 have taken place.

The browser saw 8 million downloads within the first 24-hours of its release, more than ever downloaded in a single day before and a statistic currently being considered for inclusion within the Guinness Book of Records.

Firefox 2 was downloaded 1.6 million times in its first 24 hours of release; to date, it has been downloaded more than half a billion times, according to Mozilla.

More Here

Also: My Experiences with Firefox 3.0 on Linux, Windows and Mac OS X