Editor / IDE integration

You can use any editor or IDE to contribute to Firefox, as long as it can edit text files. However, there are some steps specific to mozilla-central that may be useful for a better development experience. This page attempts to document them.


Visual Studio Code is the recommended editor for Firefox development. Not because it is better than the other editors but because we decided to focus our energy on a single editor.


This page is a work in progress. Please enhance this page with instructions for your favourite editor.

Visual Studio Code

Go to Visual Studio Code dedicated page.



There’s C++ and Rust auto-completion support for VIM via YouCompleteMe. As long as that is installed and you have run ./mach build or ./mach configure, it should work out of the box. Configuration for this lives in .ycm_extra_conf at the root of the repo.

If you don’t like YouCompleteMe, other solutions also work, but they’ll require you to create a compile_commands.json file (see below for instructions).

Rust auto-completion should work both with the default completer (RLS, as of this writing), or with rust-analyzer.


The easiest way to integrate ESLint with VIM is using the Syntastic plugin.

In order for VIM to detect jsm files as JS you might want something like this in your .vimrc:

autocmd BufRead,BufNewFile *.jsm set filetype=javascript

mach eslint --setup installs a specific ESLint version and some ESLint plugins into the repositories’ node_modules.

You need something like this in your .vimrc to run the checker automatically on save:

autocmd FileType javascript,html,xhtml let b:syntastic_checkers = ['javascript/eslint']

You need to have eslint in your PATH, which you can get with npm install -g eslint. You need at least version 6.0.0.

You can also use something like eslint_d which should also do that automatically:

let g:syntastic_javascript_eslint_exec = 'eslint_d'


Mozilla-specific packages


See the devtools documentation that describes how to integrate ESLint into Emacs.

C/C++ Development Packages

General Guidelines to Emacs C++ Programming

The following guides give an overview of the C++ editing capabilities of emacs.

It is worth reading through these guides to see what features are available. The rest of this section is dedicated to Mozilla/Gecko specific setup for packages.

rtags (LLVM/Clang-based Code Indexing)

Instructions for the installation of rtags are available at the rtags github repo.

rtags requires a compilation database.

In order for rtags to index correctly, included files need to be copied and unified compilation files need to be created. Either run a full build of the tree, or if you only want indexes to be generated for the moment, run the following commands (assuming you’re in the gecko repo root):

To increase indexing speed, it’s best to remove unified build files and test files from database updates. This can be done by creating a ~/.rdmrc file with the following contents, with [src_dir] replaced with either the repo or build directory for your checkout:

-X */[src_dir]/*Unified*;*/[src_dir]/*/test/*;*/[src_dir]/*/tests/*

Once the rdm daemon is running, the compilation database can be added to rtags like so:

rc -J [gecko_build_directory]/compile_commands.json

Note that this process will take a while initially. However, once the database is built, it will only require incremental updates. As long as the rdm daemon is running in the background, the database will be updated based on changes to files.

irony (LLVM/Clang-based Code Completion)

Instructions on the installation of irony-mode are available at the irony-mode github repo.

irony-mode requires a compilation database.

Note that irony-mode, by default, uses elisp to parse the compile_commands.json file. As gecko is a very large codebase, this file can easily be multiple megabytes, which can make irony-mode take multiple seconds to load on a gecko file.

It is recommended to use this fork of irony-mode, which requires the boost System and Filesystem libraries.

Checking the bug to get this patch into the mainline of irony-mode is recommended, to see if the fork can be used or if the mainline repo can be used. Using the Boost version of the irony-mode server brings file load times to under 1s.

Projectile (Project Management)

Instructions on the installation of projectile are available at the projectile github repo.

Projectile comes preconfigured for many project types. Since, gecko uses its own special build system (mach), a new project type needs to be added. This can be done via adding the following elisp configuration command to your emacs configuration file.

(projectile-register-project-type 'gecko
                                  '("mach" "moz.build")
                                  "python mach --log-no-times build"
                                  "python mach mochitest"
                                  "python mach run")

Assuming projectile-global-mode is on, this will allow projectile to run the correct commands whenever it is working in a gecko repo.


Emacs comes with great integration with gdb, especially when using gdb-many-windows.

However, when gdb is invoked via mach, some special arguments need to be passed in order to make sure the correct display mode is used. To use M-x gdb with mach on firefox, use the following command:

gecko_repo_directory/mach run --debug --debugparams=-i=mi


You can generate an Eclipse project by running:

./mach ide eclipse

See also the Eclipse CDT docs on MDN.

Visual Studio

You can run a Visual Studio project by running:

./mach ide visualstudio

CompileDB back-end / compileflags

You can generate a compile_commands.json in your object directory by running:

./mach build-backend --backend=CompileDB

This file, the compilation database, is understood by a variety of C++ editors / IDEs to provide auto-completion capabilities. You can also get an individual compile command by running:

./mach compileflags path/to/file

This is how the VIM integration works, for example.