With much of the focus on Firefox lately, it's easy to forget Mozilla's other important piece of software, the Thunderbird email client. The application has been one of the most popular email desktop clients for some years now, and the release of the upcoming Thunderbird 3 is getting closer with the availability of the latest beta 3 test version. While it may not be the final beta to be released, the changes between versions are impressive.

One of the biggest components to change is the HTML rendering engine Gecko, with Thunderbird now sporting the latest 1.9.1.1 version released with Firefox 3.5.1 just last week. Overall, Mozilla boasts over 500 changes since the previous version in this latest beta, including new features and tweaks, but also security and stability issues being fixed.

The biggest feature to be introduced regards the new email tabs, which some have been craving for a while now, allowing messages to be opened in new tabs. Also, all of the tabs opened will be saved when closing Thunderbird, and restored when a new session is started. Other visual changes include a new message summary view and column headings.

However, the more subtle, but important changes have been made under the hood, providing a base for further development. “Some of the biggest work in this release has happened behind the scenes, however, and will mostly be visible to extension developers. A large tab and message list refactoring has cleaned up a lot of old cruft, and many things should be quite a bit easier going forward, though existing extensions will need to adapt. This is all in preparation for upcoming UI changes to support the new GloDa search engine,” Dan Mosedale, Mozilla CTO, wrote on his blog.

With a host of other features like improved Gmail integration and a new account setup wizard, the latest beta looks like a solid release with even bigger things bound to come as we approach the final build.

Mozilla Thunderbird 3 Beta 3 for Windows is available for download here.