Building Firefox On macOS

This document will help you get set up to build Firefox on your own computer. Getting set up won’t be difficult, but it can take a while - we need to download a lot of bytes! Even on a fast connection, this can take ten to fifteen minutes of work, spread out over an hour or two.

The details are further down this page, but this quick-start guide should get you up and running:

Quick start (Try this first!)


You will need:

  • an Apple ID to download and install Apple-distributed developer tools mentioned below

  • from 5 minutes to 1 hour to download and install Xcode, which is large

  • download and install a local copy of specific macOS SDK version

You will need administrator permissions on your machine to install these prerequisites. (You can verify that you have these permissions in System Preferences -> Users & Groups.)

See 1.1 Install Xcode and Xcode command line tools and 1.2 Get the local macOS SDK for more information on how to install these prerequisites.

Getting the source

Firstly you need to prepare a directory and get the bootstrap script that will do the rest:

# the bootstrap script needs this directory, but you can choose a different target directory for the Mozilla code later
cd ~ && mkdir -p src && cd src

# download the bootstrap script
curl -o

If you don’t have Python 3.6 or later or Mercurial installed, see 2.1a Install dependencies via Homebrew for more information on how to do so. Then in your terminal from above start the bootstrapper like this:


… and follow the prompts. This will use mercurial to checkout the source code. If you prefer to work with git, use this command instead:

python3 --vcs=git

If you don’t have Homebrew or Ports installed - software package managers that will let us install some programs we’ll need - you’ll be asked to pick one. Either will work, but most Mozilla developers use Homebrew.

If you don’t let the script clone the source for you make sure you do it manually afterward before moving onto the next step.

Build Firefox!

Now we tie it all together.

In your terminal window, cd to your Mozilla source directory chosen before and type:

# create a minimal build options file
echo "ac_add_options --with-macos-sdk=$HOME/SDK-archive/MacOSX10.11.sdk" >> mozconfig

./mach bootstrap

./mach build

The ./mach bootstrap step is a catch-all for any dependencies not covered in this documentation. If you are working on Firefox frontends or building Firefox without any changes, select Artifact Builds in the first question in ./mach bootstrap. Artifact builds will complete more quickly! Artifact builds are unsuitable for those working on C++ code.

You’re on your way. Don’t be discouraged if this takes a while; it takes some time even on the fastest modern machines and as much as two hours or more on older hardware. Firefox is pretty big, because the Web is big.

Now the Fun Starts

You have the code, you’ve compiled Firefox. Fire it up with ./mach run and you’re ready to start hacking.

Build steps (details)

Building on macOS is divided into the following steps:

  1. Install Apple-distributed developer tools - Xcode, Xcode cli tools and macOS SDK locally

  2. Install supplementary build tools

  3. Obtain a copy of the Mozilla source code

  4. Configure the Mozilla source tree to suit your needs

  5. Build Firefox

1.1 Install Xcode and Xcode command line tools

You first need to install Xcode, for which you have two options but both require you to sign in with an Apple ID:

  • From Apple Developer Download page - direct link. Install the latest release (non-beta) version of Xcode, open Xcode.xip, and then before running the extracted, move it from the download folder to /Applications. (Running it from another location may screw up various build paths, homebrew builds, etc. Fix by running sudo xcode-select -switch /Applications/ )

  • From the Mac App Store - direct link.

Open /Applications/ and let it do its initial first run and setup stuff.

Install the Xcode command line tools by running xcode-select --install in your terminal.

1.2 Get the local macOS SDK

Firefox currently requires a local copy of macOS 10.11 SDK to build (all your other apps will still use your more recent version of this SDK, most probably matching your macOS version).

There are various issues when building the Mozilla source code with other SDKs and that’s why we recommend this specific version.

To get the 10.11 SDK, first download Xcode 7.3.1 from the More Downloads for Apple Developers page. Once downloaded, mount the .dmg file. Then in the Terminal run the following:

mkdir -p $HOME/SDK-archive
cp -a /Volumes/Xcode/ $HOME/SDK-archive/MacOSX10.11.sdk

2. Install supplementary build tools

Mozilla’s source tree requires a number of third-party tools and applications to build it. You will need to install these before you can build anything.

You have the choice of how to install all these components. You can use a package manager like Homebrew or Ports. Or, you can obtain, compile, and install them individually. For simplicity and to save your time, using a package manager is recommended. The following sections describe how to install the packages using existing package managers. Choose whatever package manager you prefer.

2.1a Install dependencies via Homebrew

Homebrew is “the missing package manager for macOS.” It provides a simple command-line interface to install packages, typically by compiling them from source.

The first step is to install Homebrew. See

Once you have Homebrew installed, you’ll need to run the following:

brew install yasm mercurial gawk ccache python

Python 2 is never necessary solely to build Firefox, but it is still required for some development tasks (including testing and pushing to try). If your system does not already have a Python 2 installed, you can use brew to install one:

brew install

2.1b Install Dependencies via MacPorts

MacPorts is a package manager for macOS. If you are running Homebrew, you can ignore this section.

To install MacPorts, go to their install page, download the .dmg for your platform, and install it. If you already have MacPorts installed, ensure it is up to date by running:

sudo port selfupdate
sudo port sync

The first of these commands will ask for your root password.

Common errors include:

  • sudo doesn’t accept a blank password: create a password for your account in System Preferences.

  • port command not found: add it to your path (see the troubleshooting section below).

Use MacPorts to install the packages needed for building Firefox:

sudo port install libidl yasm python27 py27-gnureadline

You’ll then see lots of output as MacPorts builds and installs these packages and their dependencies – it takes a while, so go grab a cup of coffee.

Note: By default, this will install Python 2.7, which in turn will pull in all of the X11 libraries, which may take a while to build. You don’t need any of those to build Firefox; you may want to consider adding +no_tkinter to the install line to build a python without support for the X11 UI packages. This should result in a much faster install.

Note: With older versions of Xcode (eg 6.4) you may need to use MacPorts to get the proper version of clang, such as clang-3.6 or later. See bugs in Core, Build Config referring to clang.

2.2 Install Mercurial

Mozilla’s source code is hosted in Mercurial repositories. You use Mercurial to interact with these repositories. There are many ways to install Mercurial on macOS:

  1. Install official builds from Selenic

  2. Install via Homebrew:

brew install mercurial
  1. Install via MacPorts:

sudo port install mercurial
  1. Install via Pip:

easy_install pip && pip install mercurial

Once you have installed Mercurial, test it by running:

hg version

If this works, congratulations! You’ll want to configure your Mercurial settings to match other developers. See Getting Mozilla Source Code Using Mercurial.

If this fails with the error “ValueError: unknown locale: UTF-8”, then see the workarounds on the Mercurial wiki’s Unix Install page.

When trying to clone a repository you may get an HTTP 500 error (internal server error). This seems to be due to something that Mac Mercurial sends to the server (it’s been observed both with MacPort and Mercurial binaries). Try restarting your shell, your computer, or reinstall Mercurial (in that order), then report back here what worked, please.

3. Obtain a copy of the Mozilla source code

You may want to read Getting Mozilla Source Code Using Mercurial for the complete instructions.

If you are interested in Firefox development only then run the following command, which will create a new directory, mozilla-central, in the current one with the contents of the remote repository.

Below command will take many minutes to run, as it will be copying a couple hundred megabytes of data over the internet.

hg clone
cd mozilla-central

(If you are building Firefox for Android, you should now return to the Android build instructions.)

4. Configure the build options

In your checked out source tree create a new file, mozconfig, which will contain your build options. For more on this file, see Configuring Build Options.

To get started quickly, create the file with the following contents:

# Define where build files should go. This places them in the directory
# "obj-ff-dbg" under the current source directory
mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/obj-ff-dbg

# Enable debug builds
ac_add_options --enable-debug

# Use the local copy of specific version of macOS SDK compatible with Mozilla source code
ac_add_options --with-macos-sdk=$HOME/SDK-archive/MacOSX10.11.sdk

Firefox no longer builds with gcc 4.8 or earlier, but the build system should automatically select clang if it is available in the PATH. If that is not the case, you need to set CC and CXX. For instance, if you installed Clang 9 via Homebrew, then you need to have this in your mozconfig:


5. Build

Once you have your mozconfig file in place, you should be able to build!

./mach build

If the build step works, you should be able to find the built application inside obj-ff-dbg/dist/. If building the browser with --enable-debug, the name of the application is To launch the application, try running the following:

./mach run

Note: The compiled application may also be named after the branch you’re building; for example, if you changed these instructions to fetch the mozilla-1.9.2 branch, the application will be named or

Hardware requirements

There are no specific hardware requirements, provided that the hardware accommodates all of the software required to build Firefox. Firefox can take a long time to build, so more CPU, more RAM and lots of fast disk space are always recommended.

  • Processor: Intel CPUs are required. Building for PowerPC chips is not supported.

  • Memory: 2GB RAM minimum, 8GB recommended.

  • Disk Space: At least 30GB of free disk space.

Software requirements

  • Operating System: macOS 10.11 or later. It is advisable to upgrade to the latest “point” release by running Software Update, found in the Apple menu. You will need administrative privileges to set up your development environment

  • Development Environment: Xcode. You can obtain this from the App Store.

  • Package Management: Either Homebrew or MacPorts.

These options are specific to Mozilla builds for macOS. For a more general overview of build options and the mozconfig file, see Configuring Build Options.

  • Compiler: Firefox releases are no longer built with gcc-4.8 or earlier. A recent copy of clang is needed.

    • There are some options on where to get clang:

      • Newer versions of Xcode.

      • Following the instructions in the clang website for information on how to get it.

      • Using some of the package managers (see above).

    • Once clang is installed, make sure it is on the PATH and configure should use it.

The following options, specified with ac_add_options, are lines that are intended to be added to your mozconfig file.

  • macOS SDK: This selects the version of the system headers and libraries to build against, ensuring that the product you build will be able to run on older systems with less complete APIs available. Selecting an SDK with this option overrides the default headers and libraries in /usr/include, /usr/lib, and /System/Library.

    ac_add_options --with-macos-sdk=/path/to/SDK

    Official trunk builds use MacOSX10.11.sdk. Check build/macosx/universal/mozconfig.common for the SDK version used for official builds of any particular source release.

    Applications built against a particular SDK will usually run on earlier versions of macOS as long as they are careful not to use features or frameworks only available on later versions. Note that some frameworks (notably AppKit) behave differently at runtime depending on which SDK was used at build time. This may be the source of bugs that only appear on certain platforms or in certain builds.

For macOS builds, defines are set up as follows:

  • XP_MACOSX is defined

  • XP_UNIX is defined

  • XP_MAC is not defined. XP_MAC is obsolete and has been removed from the source tree (see {{ Bug(281889) }}). It was used for CFM (non-Mach-O) builds for the classic (pre-X) Mac OS.

This requires care when writing code for Unix platforms that exclude Mac:

#if defined(XP_UNIX) && !defined(XP_MACOSX)


  • If configure (or generally building with clang) fails with ``fatal error: ‘stdio.h’ file not found``: Make sure the Xcode command line tools are installed by running. xcode-select --install.

  • For inexplicable errors in the configure phase: Review all modifications of your PATH in .bash_profile, .bash_rc or whatever configuration file you’re using for your chosen shell. Removing all modifications and then re-adding them one-by-one can narrow down problems.