In financial terms, a billion might not seem that much nowadays, but it's still an impressive number. And it's a milestone that Mozilla expects Firefox to pass today.
It's expected that the billionth download of the Firefox browser will happen today. You can keep an eye on downloads here, or via the Twitter feed. The Firefox Dowload Guesstimator predicts that the billionth download will happen at around 15:00 GMT.
Only downloads are counted here. Updates downloaded by the browser automatically don't count, and the number covers all versions of Firefox since 2004.
[UPDATE: The billionth download happened some time between 14:46 GMT and 15:00 GMT.]