Mozilla has officially announced the availability of the second Firefox 3.1 alpha. This release includes support for the highly-anticipated HTML 5 "video" element and a handful of other features that move the browser forward.

Mozilla has been planning the 3.1 roadmap since before the launch of Firefox 3. Version 3.1 is expected to provide a strong incremental improvement and will include many of the features that were deferred for various reasons during the 3.0 development cycle. The developers are refining the user interface for tabs and tagging and are also working on on important backend improvements to boost compatibility with emerging web standards like CSS 3 and HTML 5.

The latest alpha—codenamed Shiretoko—includes support for the new video element, one of the most highly-anticipated features of HTML 5. We first looked at the video element in Firefox last year when a developer produced a preliminary implementation. At the time, the feature was highly experimental and could only be tested in a specially-patched build. The feature didn't make into the main tree in time for Firefox 3, but now it's a high priority feature for the 3.1 release.

The HTML 5 video element offers some unique capabilities that can't presently be achieved with Flash-based video players. For instance, it enables web developers to more seamlessly intersperse video with other web content, manipulate video playback with JavaScript, and access video elements directly through the document object model (DOM).

One of the most compelling demos shows translucent videos playing inside of SVG frames that a user can drag and resize. The demo is implemented entirely using standards-based web technologies and shows how web developers could soon deliver rich interactive video content without Flash.

Another significant feature that landed in this alpha release is support for web worker threads, a new scripting capability that allows computationally intensive JavaScript to be run in the background so that it doesn't cause the Firefox user interface to hang. This feature and the significant JavaScript performance boost brought by the new TraceMonkey engine (which is still under development isn't included in alpha 2) will give developers the ability to leverage client-side processing to create web applications of unprecedented sophistication.

Other, minor features were added in alpha 2 as well, including much-improved support for dragging tabs between windows. Dragging a tab from one window to another in alpha 2 will no longer cause the page to reload. This means that users can seamlessly move tabs between windows without disrupting page state or the contents of web forms.

Firefox 3.1 is shaping up to be an impressive release. The latest features in alpha 2 build on the intriguing user interface changes and visual tab switcher that were added in the previous alpha. Users can download the new version from the Mozilla web site.