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 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 pages for Windows, Mac, and Linux.

Browser speed remains an important point of comparison. But as the five major browsers have developed over the past year, their speed differences have become more muddled. For example, Mozilla noted that when Firefox 3 was released, it took "60 milliseconds to change Gmail from showing one message to another with Firefox 3... compared with 413 milliseconds for IE 7 and 227 for Firefox 2."

Current browser benchmarks that look only at JavaScript place them all in the same ballpark now, so the point of comparison has begun to shift to graphics processing unit (GPU) hardware acceleration. This allows the browser to shove certain rendering tasks onto the computer's graphics card, freeing up CPU resources while making page rendering and animations load faster. These tasks include composition support, rendering support, and desktop compositing, and there are few benchmarks that are capable of testing it.

One interesting publicly available benchmark is the new JSGameBench from Facebook, which looks to test HTML5 in real-world gaming situations. The Firefox 4 beta was the fastest tested without WebGL and was the second fastest with it. Mozilla's own tests put Firefox 4 at three to six times faster than Firefox 3.6.

Mozilla remains a leader in developing the Web, and interestingly that role has led it to hold back on building out one of the more interesting minor features in Firefox 4. The new do-not-track feature supports a header on Web sites that tells sites and advertisers not to track you, so you don't see targeted ads as often. Internet Explorer also supports the header, and it includes robust, configurable support for blocking ad trackers; Firefox 4 relies on add-ons like AdBlock Plus to gain the list blocking.

Johnathan Nightingale, director of Firefox development at Mozilla, explained during a conversation last week at CNET's San Francisco office that Mozilla is more concerned with the larger problem of why ads were targeted in the first place.

"Beyond blocking the ad loads, which you can do with add-ons, this is a business-social trust situation between sites and users. We need people to vote with their feet, or at least want to have that conversation. We've spoken to a lot of advertisers. And by and large, they want to be good citizens here," Nightingale said. As a current solution, though, that makes users entirely dependent on advertiser behavior, which is likely to fall short of what people want.

Another security repair in Firefox 4 fixes a hole that affected all browsers until last summer--a vulnerability so old that it was mentioned in the documentation for CSS2 a decade ago. The exploit is a CSS sniffing history attack, in which malicious code can gain access to your browser history by manipulating link appearance and style. What made the bug so difficult to repair is that the simplest solution--to prevent all link style manipulation--would be like throwing the baby out with the bathwater, Nightingale had said in an interview at Black Hat 2010.

Nightingale also addressed other changes in Firefox 4 as providing the feature in question without playing fast and loose with a user's data. Firefox 4 removes the "lucky" automatic search results from the location bar's search functionality because Mozilla had "concerns about sending a lot of private data from the location bar to search engines. We will get there," he added, "but like with Sync we want to do it right."

Sync is another new feature in Firefox 4 and is possibly one of the best implementations of the feature across the competition. Not only can you synchronize your data across traditional PC versions of Firefox, but you also can sync your bookmarks, passwords, preferences, history, and tabs with your Android or Maemo-running phone or tablet. However, Sync debuted in 2008 as an add-on and had a notably rough beginning. Fortunately for user data, which it used to delete seemingly at will, Mozilla fixed the problems with it.

Along with the Android support, Sync gets two security features right. One is that Firefox encrypts your data before sending it over an encrypted connection to its servers, where it remains encrypted. Mozilla said it could not access the data even if somebody there wanted to. The second is that you have the option of setting up your own personal sync server. In an age in which private data stored by corporations gets hacked and stolen with shocking regularity, setting up a personal sync server is one way to ensure that you bear the responsibility for your own data. The only problem with the feature is that it doesn't yet support syncing add-ons, a factor that is at least partially tied to Firefox's nascent restart-less add-on network, also debuting in version 4.

Other big changes in Firefox 4 include a minimalist interface with a condensed menu button that closely resembles that of Opera 11 and Chrome 10; app tabs; tab groups for keeping tabs organized; an overhauled add-on manager that also supports restart-less add-ons; and expansive support for HTML5, CSS3, and the aforementioned hardware acceleration for Direct2D and Direct3D on Windows, OpenGL on Mac, and XRender on Linux.

One "future-Web" tech that Nightingale said probably won't come to Firefox before version 5 is support for WebSocket. "The specification had security problems, so we turned it off," he said. He added that users can enable it at will through the "websocket" options in about:config.

Although it took more than two and a half years for Firefox 4 to get here, expect that time to get axed like a tree in a rainforest for Firefox 5. Mozilla plans to put Firefox on an accelerated release schedule, much like Google has done with Chrome.

Check out CNET's full review of Firefox 4 for Windows, Mac, and Linux.