Thunderbird 3 Alpha 1, code named Shredder, is available from the Mozilla site. It’s highly unstable and far from complete (hence why it’s in Alpha) so I don’t encourage you consider it for normal usage. Also beware that if you have a previous version of Thunderbird, installing Shredder might overwrite it. However it’s worth trying it out to see some of the new features we can expect in Thunderbird’s next iteration. That’s what I did.
First thing, under the hood, Thunderbird 3 has the same engine as Firefox 3 and for Mac users it now runs under Cocoa, whereas the previous version was on Carbon. The mail client integrates Apple’s Address Book, so importing and exporting your previous contacts is a snap. Similarly, the Add-ons manager is much improved, just like in the Firefox 3 Beta. It now features ‘Get Add-ons’, extensions, themes and plugins. You can search for new add-ons straight from the manager without opening up your browser.
Shredder also includes tabbed browsing, which should come in handy if you need to have many emails open and want to keep them all under the Thunderbird interface. Eager tester and developers will also find an improved Crash Reporter and a version of DOM Inspector. You can check out the release notes for more details.
One other small point: the next version of the Mail client will only work on Windows 2000 and higher. 95 and 98 users will just have to upgrade their OS if they still want to keep using Mozilla’s open source mail application.