Firefox set a new Guinness World Record for the largest number of software downloads in 24 hours.
Over 8 million people downloaded Firefox 3 in the period following its official launch, the Mozilla said in a statement.
Key rivals to Firefox are market leader Microsoft's Internet Explorer and Apple's Safari browser.
Mozilla is in a battle with Microsoft, which unveiled an experimental version of its Internet Explorer 8 in Las Vegas in April and has been looking to expand its presence on the Web.
Firefox 3's additions boost security and allow users to run Web sites when they are not connected to the Internet.