The secret is: there was no prior world record for software downloads.  Firefox invented it.  The strategy was amazing.  The press and bloggers wanted to cover it, and users had an incentive to download it as quickly as possible.  They were able to multiply the visiblity of the launch many times over traditional marketing.

They also involved the existing community quite well.  As far as grassroots efforts go, they made it easy by having toolkits for the “faithful” to spread the word.

Firefox also added an element of pride.  Downloaders could print certificates for their particpation in the occasion.

From every aspect, the launch was an amazing use of publicty and community involvment.  I applaud them for setting the record they created…  and acheiving a very critical strategic goal with their innovative marketing.  They got beyond the functional improvements of the browser and made it an event to get a new piece of software.