Justin Scott writes, “With the release of the first beta of Firefox 3.1 comes everyone’s favorite release-time festivity: extension compatibility updates! If you’re an extension developer using a maxVersion of 3.0.* or less, please test your extension before declaring 3.1b1 compatibility. Some of the changes for extension developers are listed [at the Mozilla Developer Center]. 3.1.b1 is an allowed version on AMO, but 3.1.* will not be added until closer to final release. Keep in mind that you can always look at the Developer Statistics Dashboard to see how many of your users are on 3.1 betas and may be marked as incompatible/disabled.” For more information you should check out Justin’s blog post.

The second alpha version of Firefox 3.1 is now available for download. Firefox 3.1 Alpha 2 is built on a pre-release version of the Gecko 1.9.1 platform, and it is important to note that this release is intended for developers and testers only.

Firefox 3.1 Alpha 2 introduces several new features to the browser, including: support for the HTML5 “video” element, initial support for web worker threads, support for dragging and dropping tabs between browser windows, a new selector to create areas of Aero-style “glass” in XUL, support for new CSS 2.1 and CSS 3 properties, performance improvements and new preference values for color management profile support.

Please note that this alpha release of Firefox 3.1 does not include the new TraceMonkey JavaScript engine — TraceMonkey is available in nightly builds and is currently disabled by default. For more information see the TraceMonkey wiki page.

If you’re planning to download and test Firefox 3.1 Alpha 2, you should first read the release notes and the Firefox 3.1 for developers article. More information and download links are available in the DevNews release announcement.