Mozilla has announced the availability of the third beta release of Firefox 3.1. This beta irons out some bugs and moves the browser a step closer towards the final release.

Firefox 3.1 was originally intended to be an incremental update targeted for release in late 2008. Plans changed, however, and the feature set grew significantly. As a result of the delays and the large number of noteworthy improvements that are now included in the code base, the developers are presently working on changing the version number from 3.1 to 3.5.

The 3.1 release will bring a lot of very impressive changes, including performance enhancements and new features for users and developers. It includes Firefox's new high-performance TraceMonkey JavaScript runtime which significantly boosts the speed of script execution. In addition to faster performance, the JavaScript engine has also gained support for background processing with the new worker threads feature. Native JSON parsing support will also increase performance for certain kinds of dynamic Web applications.

Firefox 3.1 delivers richer multimedia with support for the new HTML5 video element which is powered by a built-in Ogg codec. There are also some new rendering effects like support for using SVG filters on HTML. New CSS3 features implemented in 3.1 include shadows and border images.

Many of these features were demonstrated during an impressive presentation by Chris Blizzard at the Southern California Linux Expo. The most impressive demo, which was created by Mozilla's Paul Rouget, showed how JavaScript can be used to detect motion in HTML5 video content. You can try this demo yourself with the new beta release.

Rouget's latest demo shows how JavaScript can be used to render subtitles on a video. The subtitles, which are in srt format and are included in three languages, are retrieved with XMLHttpRequest and are then parsed in JavaScript and displayed correctly at the proper times.

Firefox 3.1 beta 3 can be downloaded from the Mozilla website. It's not quite ready for mainstream adoption yet, but it's definitely robust enough at this stage to be used on a day-to-day basis by developers and testers who don't mind the occasional glitch. I've been using the nightly builds on my main desktop computer. If you plan on testing it out and then switching back to Firefox 3, you should be sure to create a separate profile.

Mozilla's quality assurance team is planning a special test day tomorrow and is looking for volunteers to help out by participating in the effort. For more details about the beta, check out the official release notes.