There isn’t yet a Guinness World Record for most software downloaded in a single day, so Mozilla will truly have rewritten the record books if it manages to set one. The group’s hoping for 5 million downloads–not an unreasonable figure, given that Firefox 2 was downloaded 1.6 million times the first day of its release. Indeed, Mozilla’s already well on its way to reaching that goal: 1,721,775 users had already pledged to download the browser upon its release today at 10 a.m. PDT.

What can they expect from Firefox 3? More sophisticated bookmarking, endless customization possibilities, an “awesome bar,” more robust malware protection and a browsing experience that Mozilla claims is seven times speedier than Microsoft’s (MSFT) Internet Explorer. Said Walt Mossberg, “Firefox 3.0 is the best Web browser out there right now” and that it “tops the current versions of both IE and [Apple's (AAPL)] Safari in features, speed and security.”

Clearly, Firefox 3.0 is a force to be reckoned with now more than ever before. As of May, the browser’s worldwide market share was 18.4%, while Internet Explorer’s stood at 73.8% according to Web metrics company Net Applications. Where will it stand after setting that world record?