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 10. It is advisable to have Windows Update be fully up-to-date. See Supported Build Hosts for more information.

1. 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.

# Using the C:\mozilla-build\start-shell.bat shell from step 1:
cd c:/
mkdir mozilla-source
cd mozilla-source


When running there will be a UAC (User Account Control) prompt for PowerShell after selecting the destination directory for the source code clone. This is necessary to add the Microsoft Defender Antivirus exclusions automatically. You should select Yes on the UAC prompt, otherwise you will need to follow some manual steps below.


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 select one of the Artifact Mode options. If you are building Firefox for Android, you should also see the GeckoView Contributor Guide.

Ensure antivirus exclusions

Microsoft Defender Antivirus and some third-party 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 the following folders to your third-party antivirus exclusion list:

  • The C:\mozilla-build folder.

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

  • The %USERPROFILE%/.mozbuild directory (probably C:\Users\<user>\.mozbuild).

The script attempts to add the above folders to the Microsoft Defender Antivirus exclusion list automatically. You should check that they were successfully added, but if they’re missing you will need to add the exclusions to Microsoft Defender Antivirus manually.


If you are using Mercurial and 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.

If you are using Git and you’re already missing files (you’ll see them listed in git status), you can have them brought back by discarding changes in your source tree: git restore ..

1. Build

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

cd c:/mozilla-source/mozilla-unified
hg up -C central
./mach build

🎉 Congratulations! You’ve built your own home-grown Firefox! You should see the following message in your terminal after a successful build:

Your build was successful!
To take your build for a test drive, run: |mach run|
For more information on what to do now, see

You can now use the ./mach run command to run your locally built Firefox!

If your build fails, please reference the steps in the Troubleshooting section.

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.


If you’d like to interact with Mach from a different command line environment than MozillaBuild, there’s experimental support for it described over here.


Build errors

If you encounter a build error when trying to setup your development environment, please follow these steps:
  1. Copy the entire build error to your clipboard

  2. Paste this error on in the text area and change the “Expire in one hour” option to “Expire in one week”. Note: it won’t take a week to get help but it’s better to have the snippet be around for a bit longer than expected.

  3. Go to the introduction channel and ask for help with your build error. Make sure to post the link to the snippet you created!

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 MozillaBuild or the Firefox source contain 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.

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.