For the latest version of Firefox, the Mozilla team has announced that they’ll be attempting to set a world record for the most software downloaded in one day. They haven’t set a date yet, but it sounds like it will be happening sometime in June.

Download Day - English

Update (albeit from a few months ago):

Thanks to the support of the always amazing Mozilla community, we now hold a Guinness World Record for the most software downloaded in 24 hours. From 18:16 UTC on June 17, 2008 to 18:16 UTC on June 18, 2008, 8,002,530 people downloaded Firefox 3 and are now enjoying a safer, smarter and better Web.