In comm-central, there are three overarching directories [1]: mailnews, mail, and suite. The first directory is essentially all of the backend logic corresponding to the implementation of mail protocols, database logic, etc. The latter two directories are Thunderbird- and SeaMonkey-specific code, and almost entirely consist of the GUI display and GUI driver code, although there are some product-only components.

[1] There's a few more directories that are split out of mailnews, due more to historical reasons than any sane code split.