# 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 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 ` of the sources or by checking out the current sources from {ref}`the repository `. 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 ` 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 ` and {ref}`user ` 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 ` 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 `. 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 `, 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 ). ## parser/htmlparser The legacy HTML parser that's still used for . 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.