We thought with Mozilla's backing of the new Spicebird, that Thunderbird development was being abandoned via Mozilla. Apparently not:

There is quite alot of info on the Thunderbird 3 Mozilla Wiki Page
Thunder Bird 3 - Goals of the Thunderbird 3 Release
The 3.0 release of Thunderbird has a few high-level goals:

• To ensure that Thunderbird keeps up with the evolution of the Mozilla platform
  
• To increase adoption of Thunderbird
  
• To improve usability
  
• To facilitate the development, dissemination and use of extensions

David Ascher posts in the moz.dev newsgroup:

"... I propose:

Goal: to have at public milestone build of Thunderbird 3 in 2008.
Thunderbird 3's overall aim is to significantly grow its user base
worldwide, as well as build a strong foundation for later Thunderbird
releases.

Release-defining features:
- an integrated calendaring feature, based on Lightning
- a better search experience, especially for message content searches
- a better overall user experience

"The long-term roadmap of Thunderbird is still in flux, but there are
four high-level points which drive my thinking about Thunderbird 3:

1. Thunderbird's impact is proportional to its user count. Thus driving
adoption is my primary concern. Our current user base is very
significant (many millions of mostly quite satisfied users), but the
number of possible users of Thunderbird is orders of magnitude greater
than our current reach.

2. The reasons why people don't choose to use Thunderbird are varied,
but two primary reasons appear to be: the lack of a built-in calendar
integration (compared to Outlook for example), or a search experience
that doesn't match that offered by competitors (gmail and Mail.app for
example).

3. In addition, Thunderbird's codebase has a fair bit of technical debt
due to insufficient resourcing over the years, which has led to a
codebase which has too many scary bits, not enough test coverage, and
isn't yet able to leverage the ongoing platform improvements. In
addition, while communications clients are by nature great targets for
extension authors, the current codebase isn't extension-friendly enough,
making it too hard to build installation-specific features or experiment
with new feature ideas.

4. A fair number of Thunderbird changes have already landed on trunk,
including some important bug fixes, by a variety of contributors.
There's appropriate pressure to ship an update to Thunderbird 2 to take
advantage of those and of the platform improvements."

Read the full post @ moz.dev.planning group

Click to view attachment
Mozilla Messaging Starts Up Operations
New open source organization kicks off development of Thunderbird 3

MOUNTAIN VIEW, CALIF. - February 19, 2008 - Mozilla Messaging, the new mail focused subsidiary of the non-profit Mozilla Foundation, today announced that it has begun operations: http://www.mozillamessaging.com/en-US/news/

The initial focus for Mozilla Messaging is the development of Thunderbird 3, which will deliver significant improvements, notably integrated calendaring, better search and enhancements to the overall user experience. Thunderbird is a free, open source email application that is used by millions of people around the world and is built using the same open source development model as the award-winning Mozilla Firefox Web browser.

Mozilla Messaging has staffed a small product development team who will work as part of a community of contributors from around the world.

"We're excited to renew the focus of our open source community on the future of Thunderbird," said David Ascher, CEO, Mozilla Messaging. "Every one of us is committed to building a great email product that people will love to use and that serves as the foundation for choice in a critical area of Internet software."

Mozilla Messaging is a wholly owned subsidiary of the Mozilla Foundation. The board of directors consists of David Ascher, CEO, Mozilla Messaging; Christopher Beard, VP and General Manager, Mozilla Labs; and Marten Mickos, CEO of open source database vendor MySQL AB.

"Thunderbird email is a vital part of the Mozilla project which brings freedom to hundreds of millions of internet users worldwide," said Marten Mickos, CEO of MySQL AB. "By joining the board of Mozilla Messaging, I hope to help the world communicate better."

For more information, please see the FAQ and the following blog post from David Ascher:

FAQ: http://www.mozillamessaging.com/en-US/news/faq/2008-02-19-01
David Ascher's blog : http://ascher.ca/blog/2008/02/19/mozilla-messaging/

Thunderbird 3.0 to begin ascent next month: what to expect
The Thunderbird development community aims to release the first alpha of Thunderbird 3 next month. Plans began to take shape yesterday during the very first Thunderbird weekly status call meeting, at which developers discussed potential features and other issues of relevance.

Thunderbird 3 will use Gecko 1.9, a new version of the rendering engine that serves as the foundation for the Mozilla platform. Gecko 1.9, which has also been instrumental in the making of Firefox 3, offers a number of very significant improvements, including a new Cairo-based rendering backend and support for JavaScript 2. Improving the Thunderbird user interface is another very high priority for version 3. Plans include a visual refresh and refactoring the configuration interface to improve basic usability. Ascher believes that a better out-of-the-box experience and support for calendaring are the two killer features that will make Thunderbird a success. Resolving usability problems with the configuration system is one way that he thinks Thunderbird could be made more accommodating for new users.

This story at: ars technica

First Alpha of Next version of Thunderbird available - FYI
(for your info only - Alpha is in early development stage and for the development community only)

"We're happy to announce the release of the first of many early releases of the next version of Thunderbird.

This release, called Shredder a1 to emphasize its early nature and the need for caution, is available from the mozillamessaging.com for testers, extension developers, and other people who are curious to follow the development of the next release of Thunderbird.

This release has few functionality changes compared to the current release of Thunderbird (2.0.0.14), but it is built on the same platform as Firefox 3. We expect to release frequent early releases as we work towards a major new release of Thunderbird. "

Release Notes give an idea of what's upcoming.

FYI - Thunderbird 3 beta 1 code freeze now in effect (Wed, 24 Sep 2008)
@ mozilla groups mailing list
Hopefully, we'll see Beta 1 for public testing soon...