Firefox 3 is going be released in a few days. In order to create a buzz for the release, they have created an initiative called "Download Day".
The goal is simple, you sign up, and you promise to download the software within the first 24 hours of its release so that they can beat the record for most downloads in a day and put the software in the Guinness Book of World Records.
The details are available at the following page:
http://www.spreadfirefox.com/en-US/worldrecord
[translation by jwa]