Building Firefox On Windows

This document will help you get set up to build Firefox on your own computer. Getting set up 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.

If you’d prefer to build Firefox for Windows in a virtual machine, you may be interested in the Windows images provided by Microsoft.


  • Memory: 4GB RAM minimum, 8GB+ recommended.

  • Disk Space: At least 40GB of free disk space.

  • Operating System: Windows 7 or later. It is advisable to have Windows Update be fully up-to-date.

1. System preparation

1.1 Install Visual Studio

Download and install the Community edition of Visual Studio 2019. If you have the Professional or Enterprise edition, that is also supported.

Ensure you’ve checked the following items for installation:

  • In the Workloads tab:
    • Desktop development with C++.

    • Game development with C++.

  • In the Individual components tab:
    • Windows 10 SDK (at least 10.0.17134.0).

    • C++ ATL for v142 build tools (x86 and x64).

1.2 Install MozillaBuild

Install MozillaBuild.

Accept the default installation directory. Windows may prompt you to “reinstall with the correct settings”, which you should click to accept.

When working with Firefox tooling, you’ll need to do so from within the MozillaBuild shell. You can start it by running C:\mozilla-build\start-shell.bat (you may want to make a shortcut to this file so it’s easier to start).


The MozillaBuild shell is a lot more like a Linux shell than the Windows cmd. You can learn more about it here.

2. Bootstrap a copy of the Firefox source code

Now that your system is ready, we can download the source code and have Firefox automatically download the other dependencies it needs. The below command will download a lot of data (years of Firefox history!) then guide you through the interactive setup process.

cd c:/
mkdir mozilla-source
cd mozilla-source
curl -O


In general, the Firefox workflow works best with Mercurial. However, if you’d prefer to use git, you can grab the source code in “git” form by running the bootstrap script with the vcs parameter:

python3 --vcs=git

This uses Git Cinnabar under the hood.

Choosing a build type

If you aren’t modifying the Firefox backend, then then select one of the Artifact Mode options. If you are building Firefox for Android, you should also see the GeckoView Contributor Guide.

Set antivirus exclusions

Windows Defender and some scanning antivirus products are known to significantly degrade build times and sometimes even cause failed builds (due to a “missing file”). This is usually because we have tests for well-known security bugs that have code samples that antivirus software identifies as a threat, automatically quarantining/corrupting the files.

To avoid this, add two folders to your antivirus exclusion list:

  • The C:\mozilla-build folder.

  • The directory where the Firefox code is (probably C:\mozilla-source).

If you haven’t installed an antivirus, then you will need to add the exclusions to Windows Defender.


If you’re already missing files (you’ll see them listed in hg status, you can have them brought back by reverting your source tree: hg update -C).

3. Build

Now that your system is bootstrapped, you should be able to build!

cd mozilla-unified
./mach build
./mach run

🎉 Congratulations! You’ve built your own home-grown Firefox!

Now the fun starts

Time to start hacking! You should join us on Matrix, say hello in the Introduction channel, and find a bug to start working on. See the Firefox Contributors’ Quick Reference to learn how to test your changes, send patches to Mozilla, update your source code locally, and more.


MozillaBuild out-of-date

The build system expects that you’re using the most-recent MozillaBuild release. However, MozillaBuild doesn’t auto-update. If you’re running into local issues, they may be resolved by upgrading your MozillaBuild.

Spaces in folder names

Firefox will not build if the path to the installation tool folders contains spaces or other breaking characters such as pluses, quotation marks, or metacharacters. The Visual Studio tools and SDKs are an exception - they may be installed in a directory which contains spaces. It is strongly recommended that you accept the default settings for all installation locations.

Installing Visual Studio in a different language than Windows

If Visual Studio is using a different language than the system, then your build may fail with a link error after reporting a bunch of include errors.

Quotation marks in PATH

Quotation marks (“) aren’t translated properly when passed to MozillaBuild sub-shells. Since they’re not usually necessary, you should ensure they’re not in your PATH environment variable.

PYTHON environment variable

If PYTHON is set, the build may fail with the error: “The system cannot find the file specified.” Ensure that you aren’t having a PYTHON environment variable set.

Cygwin interference

If you happen to have Cygwin installed, its tools may erroneously be used when building Firefox. Ensure that MozillaBuild directories (in C:\mozilla-build\) are before Cygwin directories in the PATH environment variable.

Building from within Users

If you encounter a build failure with: LINK: fatal error LNK1181: cannot open input file ..\..\..\..\..\security\nss3.lib and the Firefox code is underneath the C:\Users folder, then you should try moving the code to be underneath C:\\mozilla-source instead.