From MozillaZine Knowledge Base

Introductory help

• Thunderbird 3.1 FLOSS manual, most current and (almost) official manual
• Managing Thunderbird mail accounts at gunnars.net (2.0 and earlier)
• Thunderbird 2 Tutorial at FreeMailTutorials
• Security in a box guide to installing Thunderbird and the Enigmail extension (OpenPGP)

Support

• MozillaZine Thunderbird support forum
• Mozilla Messaging at GetSatisfaction (official support channel)
• Thunderbird Help & Tutorials at Mozilla Messaging
• Thunderbird support newsgroup or via google, Thunderbird support newsgroup
• Thunderbird Portable (USB flash drive based) support forum

Non-English forums

• Mozilla.cz fórum (Czech)
• MozBrowser nl-be (Dutch)
• GeckoZone (French)
• Thunderbird Mail DE (German)
• Mozilla.org.il חדשות (Hebrew)
• Mozilla Italia (Italian)
• MozillaZine (Japanese)
• Mozilla Россия (Russian)
• MozillaES (Spanish)
• MozillaZine (Turkish)
• Український форум Mozilla (Ukrainian)

Other

• A list of Thunderbird communication channels
• A collection of Mozilla links
• BabelZilla (localization support forum for add-ons)
• Calendar weblog (Lightning)
• David Ascher blog (CEO of Mozilla Messaging)
• Dmose blog (Thunderbird tech lead)
• Documenting Mozilla Messaging projects blog
• Known vulnerabilities in Mozilla products
• Mozilla Add-ons blog
• Mozilla Bugzilla (bug reports)
• Mozilla Developer Center (now called the Mozilla Developer Network)
• Mozilla Downloads is an archive of old projects such as the Grendel mail client and the Netscape Messaging Access SDK which use earlier versions of the mail/news code used by Thunderbird.
• Mozilla Labs - Messaging Add-ons (add-ons that will eventually be merged into Thunderbird)
• Mozilla Error Lookup
• Mozilla Localization (L10n)
• Planet Mozilla Messaging (blog aggregator)
• Roland Tanglao blog (Mozilla Messaging support lead)
• Mozilla Messaging/Thunderbird infrastructure
• Postbox knowledge base. Postbox has a different GUI and features but has enough in common (since its based on Thunderbird) that some of the articles might be useful.
• tb-enterprise is a mailing list to discuss issues about deploying and configuring Thunderbird in enterprise environments.
• Thunderbird Enterprise wiki
• The Differences between SeaMonkey and Firefox/Thunderbird
• Thunderbird page in Mozilla Wiki
• Thunderbird:Backend Hacking Guide For Newbies
• Wikipedia article on Thunderbird
• Thunderbird cross-reference (search source code)

Build status

• An explanation about versions and builds for testing...
• Mozconfig build files
• Releases (MozillaWiki page for for tracking in-progress releases)
• The Rumbling Edge: Weekly developments in Mozilla Thunderbird builds
• Tinderbox for comm-1.9.2 tree (Thunderbird 3.1)
• Tinderbox for comm-central tree (Thunderbird 3.next)
• Thunderbird infrastructure.
• Tinderbox for try server (Thunderbird try)
• Pushlog for try server (current test)
• Viewing and searching Mozilla source code online

Status

• Download and daily active installations metrics
• Features being evaluated or worked on
• Mozilla releases
• Thunderbird project page
• Thunderbird status meetings
• What release a feature will be in

Development Plans

• Active branches
• Address book roadmap
• Thunderbird Status
• 3.1 back end plans
• Post 3.1 plans
• Features expected to work on in next few months
• Experimental feature-development add-ons
• Feedback & Ideas for Messaging Add-ons (Mozilla Labs)
• Long term vision/goals
• Mail/News (used by both Thunderbird and SeaMonkey)
• Modern Address Book
• Moving to libxul
• Standard8's (aka Mark Banner) MozillaWiki page It typically has "thoughts in progress" sections on changes being considered.
• tb-planning is a moderated mailing list to "offer an easy, transparent venue for getting constructive, Thunderbird-related work done".

Specifications

• AutoComplete
• AutoConfiguration
• Code review requirements . It also lists who is responsible for each sub-module.
• Encryption technologies
• Gloda
• HTML Sanitizer
• HTTP extensions for Account Management and Session Identification
• LDAP support
• Mail UE Specs
• Mail/News Engineering page
• Mail/News Related Standards
• Message Reader
• Message Threading
• Network Security Services (NSS)
• NSS shared database
• SSL error codes
• Pluggable mail stores
• Thunderbird 3 for developers
• Thunderbird 3.1 for developers
• Text to HTML conversion
• Using the Multiple Accounts API
• X-Mozilla-Status explained

https://developer.mozilla.org/Special:Tags?tag=Thunderbird lists all of the pages tagged with Thunderbird on the MDC web site