Three years ago, Firefox 3 set the record for most downloads in a 24-hour period, cracking 8 million and positioning itself as a viable alternative to Internet Explorer.

Firefox 4 released today to the public at large after 12 public betas, two release candidates, and nearly a year of development, faces a hugely different landscape. (Download Firefox 4 here for Windows, Mac, and Linux.) Microsoft's Internet Explorer remains the dominant browser. And in less than three years, a significant chunk of the browser market has taken a shine to relative newcomer Google Chrome.

Mozilla flips the switch from version 3.6.15 to version 4 as Firefox possesses more than 400 million active users. (Mozilla has opened a download tracker at glow.mozilla.org.) The new version of the browser sports several massive changes, including a radically redesigned interface, significantly faster browsing speeds, strong support for the still-in-development HTML5 and other "future-Web" tech, and competitive features like synchronization, restart-less add-ons, and tab grouping. You can read the official CNET reviews of Firefox 4 at the Download.com pages for Windows, Mac, and Linux.