The browser wars are back.

Just two weeks after Microsoft delivered its highly anticipated Internet Explorer 7, Mozilla has shipped a major update to its Firefox browser.

Firefox 2.0 was officially released just after 2 p.m. Pacific Time on Tuesday, a day after an early version of the software was leaked onto Mozilla’s FTP site. Firefox 2.0 can be downloaded here.

But that early version was not integrated with Mozilla’s Web and add-on sites, making Tuesday’s version the first release that offered the complete Firefox 2.0 user experience, said Mike Beltzner, a user interface expert with the open-source project.

New features in this latest version of the open-source browser include a spellchecker for Web forms, antiphishing warnings, and improved search and tabbed browsing capabilities.

Most Firefox 1.5 extensions will work with the 2.0 browser, but add-on maintainers will need to make some slight changes to let the Firefox 2.0 browser know that their software is compatible. "If you’ve got an add-on that somebody wrote three years ago that they’re no longer maintaining, then that’s maybe a bit more problematic," Beltzner said in an interview Tuesday afternoon.

The 2.0 browser has been criticized for its lack of major new features, but it does include a very large number of smaller bells and whistles, as well as many bug fixes, Beltzner said. "This is a very different product from Firefox 1 or 1.5," he said.

Downloads of the software had been "incredibly smooth," Beltzner said. He expects "a large bulk of our existing user base to move themselves to version 2 in the next couple of weeks."

Firefox users expecting their browser to automatically install version 2.0 may have to wait a little longer. Firefox’s automatic update feature does not presently give users the option of declining an update. And because some users may not want to move to version 2.0, Firefox will first ship a 1.5.0.8 upgrade to the browser that will give them the ability to decline the major upgrade. After that it will push out the 2.0 version via automatic update.

This 1.5.0.8 release will also include some bug fixes, Beltzner said. It is expected to be delivered by the end of next month, he added.

November is also when the Mozilla developers plan to begin thinking of the next major upgrade of their browser. But after pulling long hours to get the 2.0 release out the door, some of the developers have more pressing plans, according to Beltzner.

"We get to take a nap," he said.