The final record breaking 8,002,530 downloads for Firefox 3.0 took place in June with parties in over 25 countries.
"The enthusiasm and creativity of Firefox fans was key to making this happen" said Marketing head Paul Kim.
Gareth Deaves of Guinness World Records called it "an extremely impressive accomplishment".
The official figure was confirmed after logs from download servers were audited and checked to ensure duplicate and unfinished downloads were not counted.
Mr Kim told the BBC: "The notion of going for a world record, as gooky and nutty as it may have sounded, was a really sticky idea.
"It was an idea that translated really well across national borders and to all different kinds of people around the world."More>>