Thunderbird: email application (and also a news client, chat client, calendar...)
subsystem
Gecko: Mozilla web rendering engine (See also the new Servo engine). Gecko's function is to read web content, such as HTML, CSS, XUL, JavaScript, and render it on the user's screen or print it. In XUL-based applications Gecko is used to render the application's user interface as well.
Mozilla Cross Platform Component Object Model: XPCOM is a cross platform component object model, similar to Microsoft COM. It has multiple language bindings, allowing XPCOM components to be used and implemented in JavaScript, Java, and Python in addition to C++. Interfaces in XPCOM are defined in a dialect of IDL called XPIDL.
Necko: Necko is a network library that provides a platform-independent API for several layers of networking, ranging from transport to presentation layers. This API is used in many Mozilla-based client applications (including Firefox) and can be used for writing other networking clients.
Docshell: The main object managing things related to a document window. Each frame has its own docshell. It 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.
Libxul: Provides a static build of "gecko" including XPCOM, networking, DOM, layout, rendering, and embedding modules in one shared library (avoids exposing many shared libraries)
mail and newsgroup: e-mail and newsgroup component used by Thunderbird and Seamonkey
techno
Mach: Mach is a build-tool used to build mozilla apps
Marionette: Marionette is a remote protocol that lets out-of-process programs communicate with, instrument, and control Gecko-based browsers.
XML User Interface Language: Mozilla's XML-based language for building user interfaces of applications like Firefox.