# Firefox Source Code Directory Structure

This article provides an overview of what the various directories contain.

To simply take a look at the Firefox source code, you do not need to
download it. You can look at the source directly with your web browser
using Searchfox (start at <https://searchfox.org/firefox-main/source> for
the complete firefox source code of branch HEAD).

In order to modify the source, you have to acquire it either by
downloading a {ref}`snapshot <Firefox Contributors' Quick Reference>` of the sources or
by checking out the current sources from
{ref}`the repository <Firefox Contributors' Quick Reference>`.

This document describes the directory structure -- i.e., directories that
are used by at least some of the
Mozilla project's client products. There are other directories in the
other Mozilla repository, such as those for Web tools and those for the
Classic codebase.

See the [more detailed overview of the pieces of Gecko](https://wiki.mozilla.org/Gecko:Overview).

## .cargo

Configuration files for the [Cargo package
manager](https://crates.io/).

## .vscode

Configuration files used by the [Visual Studio Code
IDE](https://code.visualstudio.com/) when working in the
mozilla-central tree.

## accessible

Files for accessibility (i.e., MSAA (Microsoft Active Accessibility),
ATK (Accessibility Toolkit, used by GTK) support files). See
[Accessibility](https://developer.mozilla.org/docs/Web/Accessibility).

## browser

Contains the front end code (in XUL, Javascript, XBL, and C++) for the
Firefox desktop browser. Many of these files started off as a copy of files in
[xpfe](#xpfe).

## browser/extensions

Contains [PDF.js](https://mozilla.github.io/pdf.js/) and
[WebCompat](https://github.com/mozilla/webcompat-addon) 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](#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](https://www.mozilla.org/projects/security/components/) .

## chrome

{ref}`Chrome registry <Chrome Registration>` used with [toolkit](#toolkit)/.
These files were originally copies of files in `rdf/chrome/`.

## config

More files used by the build process, common includes for the makefiles,
etc.

## devtools

The Firefox Developer Tools server and client components. See {ref}`contributor <devtools-contributor-doc>` and {ref}`user <devtools-user-doc>` documentation.

## docs

Contains the documentation configuration ([Sphinx](http://www.sphinx-doc.org/) based), the index page
and the contribution pages.

## 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.

## dom

- {ref}`IDL definitions <XPIDL>` 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.
- Implementations of a few of the core "DOM Level 0" objects, such as
  [window](https://developer.mozilla.org/docs/Web/API/Window) , [window.navigator](https://developer.mozilla.org/docs/Web/API/Window/navigator), [window.location](https://developer.mozilla.org/docs/Web/API/Window/location), etc.

## editor

The editor directory contains 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.

## 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](https://www.mozilla.org/projects/netlib/integrated-auth.html).

## extensions/pref

Preference-related extensions.

## extensions/spellcheck

Spellchecker for mailnews and composer.

## extensions/universalchardet

Detects the character encoding of text.

## 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.

## gradle

Containing files related to a Java 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 Firefox
supports.

## intl

Internationalization and localization support. See
[L10n:NewProjects](https://wiki.mozilla.org/L10n:NewProjects).

## 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/src

The JavaScript engine, also known as
{ref}`SpiderMonkey <SpiderMonkey>`.
See also [JavaScript](https://developer.mozilla.org/docs/JavaScript).

## js/xpconnect

Support code for calling JavaScript code from C++ code and C++ code from
JavaScript code, using XPCOM interfaces. See
[XPConnect](https://developer.mozilla.org/docs/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](https://www.mozilla.org/newlayout/doc/) and [other
information](https://www.mozilla.org/newlayout/).

## layout/base

Code that deals with the rendering tree.

## layout/forms

Rendering tree objects for HTML form controls.

## layout/generic

The basic rendering object interface and the rendering tree objects for
basic CSS boxes.

## layout/mathml

Rendering tree objects for [MathML](https://developer.mozilla.org/docs/Web/MathML).

## layout/svg

Rendering tree objects for [SVG](https://developer.mozilla.org/docs/Web/SVG).

## layout/tables

Rendering tree objects for CSS/HTML tables.

## layout/xul

Additional rendering object interfaces for [XUL](https://developer.mozilla.org/docs/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.

## mobile

## mobile/android

Firefox for Android and Geckoview

## 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

{ref}`Networking library <Networking>`, also known as Necko.
Responsible for doing actual transfers from and to servers, as well as
for URI handling and related stuff.

## netwerk/cookie

Permissions backend for cookies, images, etc., as well as the user
interface to these permissions and other cookie features.

## nsprpub

Netscape Portable Runtime. Used as an abstraction layer to things like
threads, file I/O, and socket I/O. See {ref}`NSPR`.

## 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
Firefox 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/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.

## parser/xml

The code for integrating expat (from parser/expat) into Gecko.

## python

Cross module python code.

## python/mach

The code for the {ref}`Mach` building tool.

## security

Contains NSS and PSM, to support cryptographic functions in mozilla
(like S/MIME, SSL, etc). See {ref}`Network Security Services (NSS)`
and
[Personal Security Manager
(PSM)](https://www.mozilla.org/projects/security/pki/psm/).

## services

Firefox accounts and sync (history, preferences, tabs, bookmarks,
telemetry, startup time, which addons are installed, etc). See
[here](https://docs.services.mozilla.com/).

## servo

[Servo](https://servo.org/), the parallel browser engine project.

## startupcache

XXX this needs a description.

## storage

[Storage](https://developer.mozilla.org/docs/Mozilla/Tech/XPCOM/Storage): XPCOM wrapper for sqlite. Wants to
unify storage of all profile-related data. Supersedes mork. See also
[Unified Storage](https://wiki.mozilla.org/Mozilla2: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

Vendored dependencies maintained outside of Mozilla.

## 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](https://developer.mozilla.org/docs/XPInstall) and
the [XPInstall project
page](https://www.mozilla.org/projects/xpinstall/).

## tools

Some tools which are optionally built during the mozilla build process.

## tools/lint

The linter declarations and configurations.
See [linting documentation](/code-quality/lint/)

## uriloader

## 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](https://www.mozilla.org/docs/docshell/uri-load-start.html)
and [The Life Of An HTML HTTP
Request](https://www.mozilla.org/docs/url_load.html).

## 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](/en-US/docs/XPCOM). Also
contains data structures used by the rest of the mozilla code. See also
[XPCOM Project](https://www.mozilla.org/projects/xpcom/).

## 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.