Building Firefox On Linux

They aren’t complicated, but there are a few prerequisites to building Firefox on Linux. You need:

  1. A 64-bit installation of Linux. You can check by opening a terminal window; if uname -m returns x86_64 you can proceed.

  2. Next, you’ll need Python 3.6 or later installed. You can check with python3 --version to see if you have it already. If not, see Installing Python. You’ll also need to install Mercurial and can do so with pip3 install Mercurial.

  3. Finally, a reasonably fast internet connection and 30GB of free disk space.

Getting Started

Getting set up on Linux is fast and easy.

If you don’t have one yet, create a “src” directory for yourself under your home directory:

mkdir src && cd src

Next download the script and save it in the src/ directory created above.


Building Firefox in Linux on top of a non-native file system - for example, on a mounted NTFS partition - is explicitly not supported. While a build environment like this may succeed it may also fail while claiming to have succeeded, which can be quite difficult to diagnose and fix.

And finally, 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 (make sure you have git installed):

python3 --vcs=git

Let’s Build Firefox

You’re ready; now we can tie it all together. In your terminal:

cd mozilla-unified  # ... or the name of the repo you chose in the above step

If you are not working on the C/C++ files you can also opt for Artifact Builds which are much faster. To enable artifact build set up a mozconfig file with the following options:

# Automatically download and use compiled C++ components:
# This option will disable C/C++ compilation
ac_add_options --enable-artifact-builds

# Write build artifacts to (not mandatory):
mk_add_options MOZ_OBJDIR=./objdir-frontend

If you plan to walk through code with a debugger, set up a .mozconfig file with the following options:

ac_add_options --disable-optimize
ac_add_options --enable-debug

Older clang versions (especially clang 6) from LTS linux distributions sometimes miscompile Firefox, resulting in startup crashes when starting the resulting build. If this happens, you can force the use of the clang version that ./mach bootstrap downloaded by adding the following to your .mozconfig:

export CC=path/to/home/.mozbuild/clang/bin/clang
export CXX=path/to/home/.mozbuild/clang/bin/clang++

And finally, run the build command:

./mach build

If you encounter any error related to LLVM/Clang on Ubuntu or Debian, download the latest version of LLVM and Clang and then re-run ./mach build.

And you’re on your way, building your own copy of Firefox from source. Don’t be discouraged if this takes a while; this takes some time on even the fastest modern machines, and as much as two hours or more on older hardware. When the --enable-artifact-builds option is used, builds usually finish within a few minutes.

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. The next steps are up to you: join us on Matrix in the Introduction channel, and find a bug to start working on.

General considerations

  1. 4GB RAM with an additional 4GB of available swap space is the bare minimum, and more RAM is always better - having 8GB or more will dramatically improve build time.

  2. A 64-bit x86 CPU and a 64-bit OS. As of early 2015 it is no longer possible to do a full build of Firefox from source on most 32-bit systems; a 64-bit OS is required. “Artifact Builds” may be possible, but are not a supported configuration. On Linux you can determine this by typing “uname -a” in a terminal. It is possible to build a 32-bit Firefox on a 64-bit system, see Building Firefox 32-bit on Linux 64-bit.

  3. A recent version of Clang is required to build Firefox. You can learn more about the features we use and their compiler support.

  4. If you are on a Fedora machine then simply install the following prerequisites from the terminal window:

sudo dnf install @development-tools @c-development gtk2-devel gtk3-devel libXt-devel GConf2-devel dbus-glib-devel yasm-devel alsa-lib-devel pulseaudio-libs-devel

Installing Python

To build Firefox, it’s necessary to have a Python of version 3.6 or later installed. Python 2 is no longer required to build Firefox, although it is still required for some development tasks, like testing and pushing to try.

Often, you can install both Python 2 and 3 with your system package manager. Make sure your system is up to date! However, users on older Linux distributions might find they are unable to install a recent enough Python 3, while users on newer Linux distributions may find that they can no longer install Python 2.7. pyenv is an easy way to install arbitrary Python versions if you fall into either of these categories. Your system package manager may or may not provide pyenv, but the pyenv GitHub repository provides detailed manual installation instructions in any case.

Once you have pyenv configured properly and pyenv’s shims directory at the front of your $PATH, you can easily install any version of Python and configure your project to use them. For example, at the root of your checkout, do the following:

pyenv install 2.7.17
pyenv install 3.7.8
pyenv local 3.7.8 2.7.17

Requirements for Debian / Ubuntu users

You need a number of different packages:

# the rust compiler
aptitude install rustc

# the rust package manager
aptitude install cargo

# the headers of important libs
aptitude install libgtk-2-dev
aptitude install libgtk-3-dev
aptitude install libgconf2-dev
aptitude install libdbus-glib-1-dev
aptitude install libpulse-dev

# rust dependencies
cargo install cbindgen

# an assembler for compiling webm
aptitude install yasm

# Python 3 dependencies. This will work on Ubuntu 18.04LTS and later or
# Debian buster and later. For earlier releases of Ubuntu or Debian, you
# may prefer to use pyenv.
aptitude install python3 python3-dev python3-pip python3-setuptools

# Python 2 dependencies. This will work on Ubuntu versions prior to 20.04 LTS
# and Debian versions prior to bullseye. For later releases of Ubuntu or
# Debian, you may prefer to use pyenv.
aptitude install python python-dev python-pip python-setuptools

One-Line Bootstrapping

Our system bootstrapping script can automatically install the required dependencies. You can download and run it by copying this line and pasting it into a terminal window:

wget -q -O && python3


Note: piping to stdin of a python process will cause interactive prompts in the bootstrap script to fail, causing the bootstrap process to fail. You must run Python against a local file.

If the above command fails, the reason is often because some Linux distributions ship with an outdated list of root certificates. In this case, you should upgrade your Linux distribution or use your browser to download the file. That ensures that you will get it from the right source. If you get an error from this process, consider filing a bug saying that the bootstrapper didn’t work and contact Mike Hoye <> directly for help. Please include the error message and some details about your operating system.

If you have already checked out the source code via Mercurial or Git you can also use Mach with the bootstrap command:

./mach bootstrap

Common Bootstrapper Failures

wget: command not found

You may not have wget (or curl) installed. In that case, you can either install it via your package manager:

On Debian-based distros like Ubuntu:

sudo apt install wget

On Fedora-based distros:

sudo dnf install wget

or you can just download using your browser and then run it with this command:


In some cases people who’ve customized their command prompt to include emoji or other non-text symbols have found that fails with a UnicodeDecodeError. We have a bug filed for that but in the meantime if you run into this problem you’ll need to change your prompt back to something boring.

More info

The above bootstrap script supports popular Linux distributions. If it doesn’t work for you, see Linux build prerequisites for more.