Thunderbird leverages a lot of Gecko, but two directories in the main mozilla tree are where the bulk of Thunderbird's functionality is included. There are two directories, one specific to Thunderbird, and one shared with SeaMonkey.
Thunderbird specific directories:
- app: icons and other requirements for app packaging
- base: main XUL/JS files
- components (many of these are Thunderbird-specific derivatives of similar files in the /mailnews directory)
- migration: code to support migration of settings and data from Dogbert, Eudora, Netscape Mail, Outlook Express, Outlook, and SeaMonkey.
- search: Spotlight integration
- shell: integration with default mail handler on various operating systems (Mac, Windows, GNOME)
- config (build configuration file)
- newsblog: RSS reader
- smime: S/MIME ???