The new version of the Firefox Web browser has become available as a free download.
The release was delayed as visitors checking for the update overloaded Firefox's Web servers.
Firefox supporters organised launch parties around the world as they tried to set a world record for most software downloads in a 24-hour period.
The category is new, and Guinness World Records must certify it, a process that could take a week or longer.
Firefox comes from Mozilla, an open-source community in which thousands of people, mostly volunteers, collectively develop free products. Firefox is the No.2 Web browser behind Microsoft's Internet Explorer.
Firefox 3 includes enhancements to help users organise their frequently visited websites and block access to sites known to distribute viruses and other malicious software.
Users of Yahoo Inc's mail service can also use Firefox 3 to send e-mail by clicking a "mailto" link they might come across clicking on a name or a "contact us" link on a Web page. Previously such links could only open a standalone, desktop e-mail programme.
Firefox 3 also offers other design and speed improvements.
