Mozilla officially released Firefox 2 on Tuesday in the US, adding security features and a new interface to the browser.

Mozilla has set up two download sites for the udpate, which it said it has optimised for the expected high volume of traffic, at Getfirefox.com and Mozilla.com/firefox.

"The key focus in Firefox 2 is making sure that what we are focusing on is tightness and efficiency -- with the ability of turning that browser into exactly what one needs, focusing on security, stability and speed," said Mike Beltzner, who Mozilla describes as its "phenomenologist".

The revamped Firefox includes a new interface theme and more security protection such as built-in phishing protection. It also has session memory, which, when the browser is re-opened, brings back the set of Web pages that were in use when it was last closed. Changes have also been made in the technology to import RSS feeds, which now offers a feed list view with title and first lines.

The camp in favour of having a 'close' button on each tab has won over the majority who argued against them, Beltzner said. Previously, there was one 'close' button at the right of the bar. Clicking on this closed only the one last viewed -- but it could be difficult to work out which one this was.

"Google did usability studies with eye-tracking tools and determined that people actually look to the tab first, and it would take longer to determine if they had the right tab and were ready to close it," Beltzner said.

"NASA Ames recently did cognitive modeling for us on tabs. Not only was 'close' button on a tab quicker, but people would be more accurate. They also gave us good data on how wide tabs had to be before people clicked on the wrong one."

The NASA results also convinced Mozilla developers that people will inevitably close the wrong tab at some point, no matter what. That led to the creation of an 'undo close tab' feature.

"It will re-open the tab you just closed, and if you had written anything into a form on the site with that Web tab, it will restore that info as well," Beltzner said.

The new theme for the Firefox 2 interface has a cleaner look, resembling Opera 9, with similar Web feed features, rounded buttons, and more opalescence, Mozilla said.

The Firefox 2 launch comes just days after Microsoft released Internet Explorer 7, the first update to its browser in almost five years.

"We see the release of IE7 as a great thing. It was in maintenance mode for several years. Our browser and others made people think that the Web browser could do something more for them, and that spurred on new (developments and interest in Web browsers)," Beltzner said.

Unlike IE7, Mozilla's Firefox 2 is available for Windows, Mac and Linux machines.

Early buzz
Buzz surrounding rumours of early access to the final version of Firefox 2 via an FTP to a Mozilla directory link went round the Internet on Monday and Tuesday, as fans and bloggers hoped to get an early look at the free software.

Mozilla earlier asked people not to attempt to download the update from a direct file link, saying that it would not guarantee "that any set of files currently found within its Web site or elsewhere will be the final release".

It also asked that users wait until the release of the browser, at which point Mozilla will have round-robin tactics in place to prevent overload.