Mozilla Thunderbird Project
2.0 Development Roadmap - outlines 2.0 plans and priorities.
Thunderbird Source Code
Thunderbird-specific source lives under "mozilla/mail", "mozilla/toolkit", and "mozilla/chrome" in the repository.
How to Build Thunderbird - instructions and configuration.
Localize Thunderbird - instructions. (Actually this is the Firefox instruction page but the same steps apply.
Address Book Project - information about the address book used by Thunderbird and Seamonkey.
Thunderbird Extensions - development notes for the new Extensions system.
Thunderbird Review Requirements - for all checkins to the mozilla/mail directory.
Toolkit Review Requirements - for all checkins to the mozilla/toolkit and mozilla/chrome directories.
Forums - A great place to talk about issues and problems with the latest developer builds. The Thunderbird leaders are active participants in the forum. Announcements regarding alphas, betas and release candidates are posted here as well.
Developer Newsgroup - a dedicated newsgroup for developers to talk about the Thunderbird project.
Wiki - for discussing future Thunderbird product and feature planning.
Nightly Builds of Thunderbird are available for interested community testers. These builds are generated automatically every night and contain the latest development work for Thunderbird. Community testers are encouraged to discuss the nightly builds and provide feedback in the Thunderbird Build Forum hosted by Mozillazine. Bugs can also be filed in Bugzilla, our public bug database.
1.5 Development Roadmap - outlines milestones and strategy in releasing Thunderbird 1.0 and 1.5.
We always need Heavy Lifters in code. If you're excited about open source technology and how it relates to e-mail, why not get involved with the Mozilla Project and Thunderbird? We're especially looking for people with skills in Mac OS X programming and Windows developers. Get started today by finding and fixing something. Instructions are not provided here since figuring out how to do all of this can be considered part of the "entry requirements". ;-)