A billion downloads, not a billion users. If there are only 200 million users and they kept up with 1 point release in 5 then you'd already have a billion downloads.
The latest trunk build automatically downloads and installs the newest nightly build every day.
I've been using firefox since version 0.5 (2002) and I've probably been running the latest-trunk for half of that time.
Conservatively, I've downloaded firefox > 1000 times. Just sayin'.
http://blog.chromium.org/2009/07/smaller-is-faster-and-safer...
I can't find a reference, but I think the "one billion" count only counts full downloads; updates installed via "Check for updates" aren't included in the count.