Mozilla has announced the availability of the first Firefox 5 beta release for Windows, Linux, Mac OS X and Android. The new desktop version includes a built-in release channel switcher and support for CSS animations. The new mobile version introduces support for the Do Not Track header and a number of other improvements.

Mozilla is transitioning to a faster-paced development model with shorter cycles between major releases. The organization is aiming to deliver three more major updates this year, bringing the Firefox version number to 7 by the end of 2011. To accommodate the more iterative release management strategy, Mozilla has established a system of release "channels" through which new improvements will flow before arriving in a stable release. It's similar to the approach already used successfully by Google for its Chrome Web browser.

The Aurora channel was created to provide a middle ground for testing features between the nightly builds and the beta releases. After being subjected to close scrutiny and testing by Aurora users for five weeks, the new Firefox 5 features have landed in a beta release. One of the new features is a combobox in the Firefox "about" dialog that allows users to select the release channel that they wish to use. The options include: stable releases, beta releases, or Aurora builds. There doesn't yet appear to be a way to get on the nightly bandwagon through the channel system yet.

Another major improvement is the addition of support for CSS animations, an experimental CSS3 feature that was originally created by Apple. It allows Web developers to use keyframe animation to tweak CSS properties. Mozilla's implementation was developed by David Baron and was merged into the Firefox code base last month. The custom "-moz" prefix is tacked onto the properties—a standard practice for CSS features that haven't been finalized—which means that you can't test it with any of the existing CSS animation demos. Baron did, however, provide a demo of his own so that users can see the feature in action.

Firefox 5 beta for Android was released concurrently with the desktop beta. The new version on Android includes support for Mozilla's Do Not Track flag, an HTTP header that is transmitted in HTTP requests to indicate to advertisers that the user doesn't wish to be tracked. Although the header isn't yet widely recognized by the advertising industry, it has started gaining some acceptance. Firefox is likely the first major browser vendor to offer the feature in a mobile port.

The desktop and mobile beta releases are available for download from Mozilla's website, alongside the latest Aurora build.