firefox-1-billionMozilla likes big download numbers. Last year, its web browser Firefox 3 set the Guinness world record for software downloads in a single day. This year, some of the team members told me they weren’t going to chase a similar record for Firefox 3.5, since it might “melt the Internet.” But now Firefox is approaching another milestone — 1 billion downloads overall.

Whether or not that’s a meaningful measure of Firefox’s market penetration is debatable, especially when the latest version of Firefox, 3.5.1, has “only” been downloaded 63 million times. Still, a 1 billion download celebration is hard to resist. In this case, it’s fun to watch Mozilla counting down through a special account on microblogging service Twitter. The most recent tweet (as of 2:50 Pacific time) says Firefox has been downloaded 999,024,648, and is currently averaging around 20.3 downloads per second.

Mozilla says it will be releasing more information about the milestone on Monday, so we can probably expect Firefox to cross the 1 billion download threshold sometime around then.

Update: Mozilla now says it will probably cross the 1 billion download mark at around 3:45am this morning (Friday).