While much of the focus at Mozilla is put on FireFox, Thunderbird is one of their most popular products that many people enjoy as a mail client. Slower to updates than other Mozilla software, many people will be happy to learn that you can now try out the version 2.0 Alpha. Of course, alpha software is strictly for testing purposes only while the team further develops the product. Definitely don't switch your mail client now, though if you are wanting to get a sneak peak there's nothing stopping you from doing such.
A beta is scheduled for release in August, with the final 2.0 release planned for “late fall” of this year. A lot of enhanced features are being added to the program, including better extension support and a function similar to grouping that makes emails appear as conversation threads. The alpha is available for download at Mozilla's site.