The mozilla source code for all projects (firefox, thunderbird) is combined into a single tree. This tree contains all the source code for all the projects in the mozilla family and all the code required to build each project on the platforms each project supports (Linux, Windows, Mac OS, etc).
Getting the Source Code
You can download a compressed file of a snapshot of the sources or you can check out the current sources from cvs.
The Source Code Directory Structure
The top level directories in the mozilla cvs repository are:
./mail ./db ./build ./nsprpub ./security ./directory ./accessible ./browser ./chrome ./ipc ./modules ./other-licenses ./storage ./toolkit ./editor ./extensions ./README ./caps ./dbm ./config ./docshell ./embedding ./gc ./intl ./jpeg ./js ./lib ./netwerk ./plugin ./profile ./rdf ./sun-java ./tools ./uriloader ./xpcom ./xpinstall ./content ./dom ./gfx ./parser ./layout ./view ./webshell ./widget ./themes