Sections of this document are out of date. The 'extensions' section for example lists many folders that have been moved into sub-directories in other parts of the codebase. A code search tool such as DXR or searchfox are the best way to discover where the code moved to. In some cases, the code was removed from mozilla-central.
The code for all projects in the Mozilla family (such as Firefox, Thunderbird, etc.) is combined into a single source tree. The tree contains the source code as well as the code required to build each project on supported platforms (Linux, Windows, macOS, etc). This article provides an overview of what the various directories contain.
To simply take a look at the Mozilla source code, you do not need to download it. You can look at the source directly with your web browser using DXR (start at https://dxr.mozilla.org/mozilla-central/source/? for the complete mozilla source code of branch HEAD).
In order to modify the source, you have to acquire it either by downloading a snapshot of the sources or by checking out the current sources from Mercurial.
This document describes the directory structure that can be pulled by client.mk -- i.e., directories that are used by at least some of the Mozilla project's client products. There are other directories in the Mozilla CVS repository, such as those for Web tools and those for the Classic codebase.
See source code directories overview for a somewhat different (older) version of the same information. Also see the more detailed overview of the pieces of Gecko.
accessible
Files for accessibility (i.e., MSAA (Microsoft Active Accessibility), ATK (Accessibility Toolkit, used by GTK+ 2) support files). See Accessibility.
addon-sdk
The add-on SDK for creating Firefox add-ons using standard Web Technologies.
b2g
Source code for the Boot2Gecko project used when building FirefoxOS.
browser
Contains the front end code (in XUL, Javascript, XBL, and C++) for the Firefox browser. Many of these files started off as a copy of files in xpfe/.
browser/extensions
Contains PDF.js and Shumway built-in extensions.
browser/themes
Contains images and CSS files to skin the browser for each OS (Linux, Mac and Windows)
build
Miscellaneous files used by the build process. See also config/.
caps
Capability-based web page security management. It contains C++ interfaces and code for determining the capabilities of content based on the security settings or certificates (e.g., VeriSign). See Component Security.
chrome
Chrome registry (See here) used with toolkit/. These files were originally copies of files in rdf/chrome/ (SeaMonkey still uses these).
config
More files used by the build process, common includes for the makefiles, etc. See also build/.
db
Container for database-accessing modules.
db/sqlite3
The SQLite database, used by storage.
devtools
XXX this needs a description.
docshell
Implementation of the 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. See also webshell/.
dom
- IDL definitions of the interfaces defined by the DOM specifications and Mozilla extensions to those interfaces (implementations of these interfaces are primarily, but not completely, in content/).
- The parts of the connection between JavaScript and the implementations of DOM objects that are specific both to JavaScript and to the DOM. (The parts that are not DOM-specific, i.e., the generic binding between XPCOM and JavaScript, live in js/src/xpconnect/.)
- Implementations of a few of the core "DOM Level 0" objects, such as
window,window.navigator,window.location, etc.
editor
The editor directory contains C++ interfaces, C++ code, and XUL/Javascript for the embeddable editor component, which is used for the HTML Editor("Composer"), for plain and HTML mail composition, and for text fields and text areas throughout the product. The editor is designed like a "browser window with editing features": it adds some special classes for editing text and managing transaction undo/redo, but reuses browser code for nearly everything else.
Contains:
- The backend for HTML and text editing. This is not only used for the mail composer and the page editor composer, but also for rich text editing inside webpages. Textarea and input fields are using such an editor as well (in the plaintext variant). See Mozilla Editor.
- The frontend for the HTML editor that is part of SeaMonkey.
embedding
This directory contains IDL, headers, and support files needed in order to embed Gecko in applications.
See also: Embedding Mozilla.
extensions
Contains several extensions to mozilla, which can be enabled at compile-time using the --enable-extensions configure argument.
Note that some of these are now built specially and not using the --enable-extensions option. For example, disabling xmlextras is done using --disable-xmlextras.
extensions/auth
Implementation of the negotiate auth method for HTTP and other protocols. Has code for SSPI, GSSAPI, etc. See Integrated Authentication.
extensions/content-packs
Content- and locale-pack switching user interface.
extensions/cookie
Permissions backend for cookies, images, etc., as well as the user interface to these permissions and other cookie features.
extensions/cview
Component viewer, which allows to view the currently registered components and interfaces.
extensions/datetime
Support for the datetime protocol.
extensions/finger
Support for the finger protocol.
extensions/gnomevfs
Interface to gnome-vfs to allow using all protocols supported by gnome-vfs inside of mozilla.
extensions/help
Help viewer and help content. See Mozilla Help Viewer Project.
extensions/irc
This is ChatZilla, the IRC (Internet Relay Chat) component. See ChatZilla.
extensions/java
XPCOM<->Java bridge ("JavaXPCOM", formerly known as Javaconnect). Unrelated to the code in java/.
extensions/layout-debug
Layout debugger. Intended as replacement for "viewer". Can be used to run layout regression tests and has support for other layout debugging features.
extensions/lightning
The Lightning project, "an extension to tightly integrate calendar functionality (scheduling, tasks, etc.) into Thunderbird." See Lightning.
extensions/mono
A two-way bridge between the CLR/.NET/Mono/C#/etc. world and XPCOM.
extensions/p3p
Implementation of W3C's Platform for Privacy Preferences standard. See Platform for Privacy Preferences (P3P).
extensions/pref
Preference-related extensions.
extensions/python
Support for implementing XPCOM components in python. See PyXPCOM.
extensions/reporter
An extension to report problems with web pages. Client-side part. See Mozilla Reporter.
extension/schema-validation
Allows to validate XML trees according to specified XML Schemas.
extension/spatialnavigation
Spatial navigation, navigating between links of a document according to their on-screen position. See Spatial Navigation.
extensions/spellcheck
Spellchecker for mailnews and composer.
extensions/sql
Support for accessing SQL databases from XUL applications. See SQL Support in Mozilla.
extensions/sroaming
Session roaming, i.e. support for storing a profile on a remote server.
extensions/transformiix
XSLT support. XSL Transformations is a language used to transform XML documents into other XML documents. See XSL Transformations.
extensions/tridentprofile
Support for importing profiles from MSIE.
extensions/typeaheadfind
Find As You Type allows quick web page navigation when you type a succession of characters in the body of the displayed page. See Find As You Type.
extensions/universalchardet
Universal character set detector.
extensions/venkman
The JavaScript Debugger.
extensions/wallet
Password and Form Manager.
extensions/webdav
WebDAV code; exposes special APIs for accessing WebDAV servers. Used by the Calendar project.
extensions/webservices
Support for Webservices. See Web Services.
extensions/xforms
Code for the XForms extension. See XForms.
extensions/xmlextras
Several XML-related extensions. See XML Extras.
extensions/xml-rpc
XML Remote Procedure Calls. Unowned these days. See XML-RPC in Mozilla.
extensions/xmlterm
XMLTerm, a terminal implemented using mozilla technology. Only available on GTK builds.
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.
gradle
Containing files related to a JVM (Java Virtual Machine) build system.
hal
Contains platform specified functions (e.g. obtaining battery status, sensor information, memory information, Android alarms/vibrate/notifications/orientation, etc)
image
Image rendering library. Contains decoders for the image formats mozilla supports.
intl
Internationalization and localization support. See L10n:NewProjects.
intl/chardet
Code for "sniffing" the character encoding of Web pages.
intl/ctl
Code for dealing with Complex Text Layout, related to shaping of south Asian languages (not built by default, needs --enable-ctl).
intl/locale
Code related to determination of locale information from the operating environment.
intl/lwbrk
Code related to line breaking and word breaking.
intl/strres
Code related to string resources used for localization.
intl/uconv
Code that converts (both ways: encoders and decoders) between UTF-16 and many other character encodings.
intl/unicharutil
Code related to implementation of various algorithms for Unicode text, such as case conversion.
ipc
Container for implementations of IPC (Inter-Process Communication).
js
js/src
The JavaScript engine, also known as SpiderMonkey. See also JavaScript.
js/jsd
JavaScript debugging library. See JavaScript Debugging.
js/xpconnect
Support code for calling JavaScript code from C++ code and C++ code from JavaScript code, using XPCOM interfaces. See XPConnect.
layout
Code that implements a tree of rendering objects that describe the types and locations of the objects that are displayed on the screen (such as CSS boxes, tables, form controls, XUL boxes, etc.), and code that manages operations over that rendering tree (such as creating and destroying it, doing layout, painting, and event handling). See documentation and other information.
layout/base
Code that deals with the rendering tree.
layout/generic
The basic rendering object interface and the rendering tree objects for basic CSS boxes.
layout/forms
Rendering tree objects for HTML form controls.
layout/tables
Rendering tree objects for CSS/HTML tables.
layout/mathml
Rendering tree objects for MathML.
layout/svg
Rendering tree objects for SVG.
layout/xul
Additional rendering object interfaces for XUL and the rendering tree objects for XUL boxes.
media
Contains sources of used media libraries for example libpng.
memory
Cross-platform wrappers for memallocs functions etc.
mfbt
Implementations of classes like WeakPtr. Multi-platform assertions etc. More on MFBT
mobile
XXX this needs a description.
modules
Compression/Archiving, math library, font (and font compression), Preferences Library
modules/libjar
Code to read zip files, used for reading the .jar files that contain the files for the mozilla frontend.
modules/libpref
Library for reading and writing preferences.
modules/zlib
Source code of zlib, used at least in the networking library for compressed transfers.
mozglue
Glue library containing various low-level functionality, including a dynamic linker for Android, a DLL block list for Windows, etc.
netwerk
Networking library, also known as Necko. Responsible for doing actual transfers from and to servers, as well as for URI handling and related stuff. See also Network library documentation.
nsprpub
Netscape Portable Runtime. Used as an abstraction layer to things like threads, file I/O, and socket I/O. See Netscape Portable Runtime.
nsprpub/lib
Mostly unused; might be used on Mac?
other-licenses
Contains libraries that are not covered by the MPL but are used in some mozilla code.
parser
Group of structures and functions needed to parse files based on XML/HTML.
parser/expat
Copy of the expat source code, which is the XML parser used by mozilla.
parser/html
The HTML parser (for everything except about:blank).
parser/xml
The code for integrating expat (from parser/expat) into Gecko.
parser/htmlparser
The legacy HTML parser that's still used for about:blank. Parts of it are also used for managing the conversion of the network bytestream into Unicode in the XML parsing case.
probes
Files related to dtrace.
python
Cross module python code.
python/mach
The code for the Mach building tool.
rdf
RDF handling APIs. See RDF and RDF. Also contains the chrome registry code used by SeaMonkey, although toolkit apps (such as Firefox and Thunderbird) use the copy forked into chrome.
security
Contains NSS and PSM, to support cryptographic functions in mozilla (like S/MIME, SSL, etc). See Network Security Services (NSS) and Personal Security Manager (PSM).
services
Firefox accounts and sync (history, preferences, tabs, bookmarks, telemetry, startup time, which addons are installed, etc). See here.
servo
At the time of writing, this directory contains placeholder information ahead of the rollout of Project Quantum's rust-bindgen.
startupcache
XXX this needs a description.
storage
Storage: XPCOM wrapper for sqlite. Wants to unify storage of all profile-related data. Supersedes mork. See also Unified Storage.
taskcluster
Scripts and code to automatically build and test Mozilla trees for the continuous integration and release process.
testing
Common testing tools for mozilla codebase projects, test suite definitions for automated test runs, tests that don't fit anywhere else, and other fun stuff.
third_party
XXX this needs a description.
toolkit
The "new toolkit" used by Thunderbird, Firefox, etc. This contains numerous front-end components shared between applications as well as most of the XBL-implemented parts of the XUL language (most of which was originally forked from versions in xpfe/).
toolkit/mozapps/extensions/test/xpinstall
The installer, which contains code for installing Mozilla and for installing XPIs/extensions. This directory also contains code needed to build installer packages. See XPInstall and the XPInstall project page.
tools
Some tools which are optionally built during the mozilla build process, mostly used for debugging.
uriloader
XXX this needs a description.
uriloader/base
Content dispatch in Mozilla. Used to load uris and find an appropriate content listener for the data. Also manages web progress notifications. See Document Loading: From Load Start to Finding a Handler and The Life Of An HTML HTTP Request.
uriloader/exthandler
Used to handle content that Mozilla can't handle itself. Responsible for showing the helper app dialog, and generally for finding information about helper applications.
uriloader/prefetch
Service to prefetch documents in order to have them cached for faster loading.
view
View manager. Contains cross-platform code used for painting, scrolling, event handling, z-ordering, and opacity. Soon to become obsolete, gradually.
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).
xpcom
Cross-Platform Component Object Model. Also contains data structures used by the rest of the mozilla code. See also XPCOM Project.
xpfe
XPFE (Cross Platform Front End) is the SeaMonkey frontend. It contains the XUL files for the browser interface, common files used by the other parts of the mozilla suite, and the XBL files for the parts of the XUL language that are implemented in XBL. Much of this code has been copied to browser/ and toolkit/ for use in Firefox, Thunderbird, etc.
xpfe/components
Components used by the Mozilla frontend, as well as implementations of interfaces that other parts of mozilla expect.
Moved/Renamed/Deleted Directories
content
The data structures that represent the structure of Web pages (HTML, SVG, XML documents, elements, text nodes, etc.) These objects contain the implementation of many DOM interfaces and also implement some behaviors associated with those objects, such as link handling, form control behavior, and form submission.
This directory also contains the code for XUL, XBL, XTF, <canvas>, as well as the code implementing XSLT and event handling.
profile
Code for profile handling, the profile manager backend and frontend.