Mozilla Source Code Directory Structure

Dirctory structure: Mozilla Source Code Directory Structure
Another: Source code directories overview

Interesting:

browser - Firefox

xulrunner - xulrunner

widget - a cross-platform API, with implementations on each platform, for dealing with operating system/environment widgets, i.e., code related to creation and handling of windows, popups, and other native widgets and to converting the system's messages related to painting and events into the messages used by other parts of Mozilla (e.g. view/ and content/, the latter of which converts many of the messages to yet another API, the DOM event API).

gfx - contains interfaces that abstract the capabilities of platform specific graphics toolkits, along with implementations on various platforms. These interfaces provide methods for things like drawing images, text, and basic shapes. It also contains basic data structures such as points and rectangles used here and in other parts of Mozilla.
It is also the home of the new graphics architecture based on cairo (via a C++ wrapper called thebes). See NewGFXAPIs and GFXEvolution.


Not so interesting:

view - view manager. Contains cross-platform code used for painting, scrolling, event handling, z-ordering, and opacity. Soon to become obsolete, gradually.

docshell - Implementation of the docshell, the main object managing things related to a document window. Each frame has its own docshell. Contains methods for loading URIs, managing URI content listeners, etc. It is the outermost layer of the embedding API used to embed a Gecko browser into an application.

webshell - Mostly obsolete, and being merged into docshell/.

IRC: irc.mozilla.org, channel #developers