Mozilla has released the first preview of Firefox 3.1, the fast-track update that the open-source company has pegged with a late 2008 or early 2009 ship date.

Code-named "Shiretoko," named for a national park on Japan's northern-most island of Hokkaido, Firefox 3.1 Alpha 1 was delayed several days because of a last-minute bug found in the Mac OS X version.

Firefox 3.1 Alpha 1 includes some, but not all, of the features Mozilla hopes to add to the update as work continues. Among the noticeable changes: several improvements to the address bar, which Mozilla dubs the "Smart Location Bar" in recognition of the search enhancements it received in Firefox 3.0; and changes to tab-switching.

Pressing Ctrl-Tab in Alpha 1 switches between current and last-viewed tabs rather than simply moving to the next tab to the right. Thumbnails of each page are also now displayed for easier recognition. Like many of the features slated for Firefox 3.1, the revised tab-switching was originally meant to be included with Firefox 3.0, but had been dropped during development to keep that June version on schedule.

Current Firefox users can add the same switching behavior to their browsers by installing the Ctrl-Tab add-on.

Firefox 3.1 Alpha 1 is the first version of the browser built on Mozilla's Gecko 1.9.1 engine, which is also in development. According to Mozilla, some of the changes in the preview will affect Web page compatibility; a "For Developers" page has been set up to spell out those changes.

Mozilla typically spins out several alpha editions before moving on to a series of betas, which in turn lead to release candidates and then to a final shipping version. Firefox 3.0, for example, ran through eight alphas, five betas and three release candidates before hitting the street in June.

However, the company has projected a much faster development cycle for 3.1. Earlier schedules slated Firefox 3.1's first beta for next month, and a final version in the last quarter of this year or the first of 2009.

The preview can be downloaded from Mozilla's site in versions for Windows, Mac OS X and Linux.