Building Firefox On Windows

Thank you for helping us build the world’s best browser on the world’s most popular OS. This document will help you get set up to build and hack on your own version of Firefox on your local machine.

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:

Getting ready

To build Firefox on Windows, you need a 64-bit version of Windows 7 or later and about 40 GB of free space on your hard drive. You can make sure your version of Windows is 64-bit on Windows 7 by right-clicking on “Computer” in your “Start” menu, clicking “Properties” and then “System”. On Windows 8.1 and Windows 10, right-clicking the “Windows” menu button and choosing “System” will show you the same information. You can alternatively press the “Windows” and “Pause Break” buttons simultaneously on your keyboard on any version of Windows.

Next, we want to start on a solid foundation: make sure you’re up to date with Windows Update and then we’ll get moving.


Microsoft is providing some Windows images with Visual Studio and code

Visual Studio 2019


As of bug 1483835, local Windows builds use clang-cl by default as compiler. Visual Studio is still necessary for build tools, headers, and SDK.


Automation builds still use Visual Studio 2017, so there may be some divergence until we upgrade. Bug 1581930 tracks various issues building with 2019. Please file your issue there and downgrade in the interim if you encounter build failures.

Download and install the Community edition of Visual Studio 2019. Professional and Enterprise are also supported if you have either of those editions.


When installing, the following workloads must be checked:

  • “Desktop development with C++” (under the Windows group)

  • “Game development with C++” (under the Mobile & Gaming group)

In addition, go to the Individual Components tab and make sure the following components are selected under the “SDKs, libraries, and frameworks” group:

  • “Windows 10 SDK” (at least version 10.0.17134.0)

  • “C++ ATL for v142 build tools (x86 and x64)” (also select ARM64 if you’ll be building for ARM64)

Make sure you run Visual Studio once after installing, so it finishes any first-run tasks and associates the installation with your account.

If you already have Visual Studio 2019 installed, you can get to the installer via its menu “Tools” → “Get Tools and Features”.

Other Required Tools


Finally, download the MozillaBuild Package from Mozilla. Accept the default settings, in particular the default installation directory: c:\mozilla-build\. On some versions of Windows an error dialog will give you the option to ‘reinstall with the correct settings’ - you should agree and proceed.

Once this is done, creating a shortcut to c:\mozilla-build\start-shell.bat on your desktop will make your life easier.

Getting the source

This is the last big step. Double-clicking start-shell.bat in c:\mozilla-build (or the shortcut you’ve created above) will open a terminal window.

Start by creating a “mozilla-source” directory off of C:\ and cd into it like so

cd c:/

mkdir mozilla-source

cd mozilla-source

Next, you can get the Firefox source code with Mercurial. There is a wiki page on how to get the source using Mercurial but, in summary, if your network connection is good enough to download 1+ GB without interuption and you want the main development repository, then you can just use:

curl -o


… and follow the prompts. This will use mercurial to checkout the source code. If you prefer to work with git, use this command instead (you’ll have to have Git for Windows installed):

python3 --vcs=git

While you’re waiting for that process to finish, take a look at our Mercurial documentation. It explains how we use version control at Mozilla to manage our code and land changes to our source tree.

Build Firefox!

Now we tie it all together. In your terminal window, cd to your source directory as before and type

cd mozilla-unified  # ... or the name of the repo you chose earlier

./mach bootstrap

./mach build

The ./mach bootstrap step is a catch-all for any dependencies not covered in this documentation. Note that, bootstrap works only with the Mercurial repo of the source, not with source tar balls, nor the github mirror. If you are working on Firefox or Firefox for Android 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++ or Rust 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.

You’re ready

When mach build completes, you’ll have your own version of Firefox built from the source code on your hard drive, ready to run. You can run it with

./mach run

Now you have your own home-built version of Firefox.

If you saw an error here, look further down in this document for the “Troubleshooting” section - some antivirus software quarantine some of our tests, so you need to create exceptions for the “mozilla-source” and “mozilla-build” directories. Don’t turn your antivirus off! Just add the exceptions.

Details and troubleshooting

Hardware and software requirements

The Firefox build process is both I/O and CPU-intensive, and can take a long time to build even on modern hardware. The minimum and recommended hardware requirements for Mozilla development are:

  • At least 4 GB of RAM. 8 GB or more is recommended, and more is always better.

  • 35 GB free disk space. This amount of disk space accommodates Visual Studio 2019 Community Edition, the required SDKs, the MozillaBuild package, the Mercurial source repository and enough free disk space to compile. A solid-state hard disk is recommended as the Firefox build process is I/O-intensive.

  • A 64-bit version of Windows 7 (Service Pack 1) or later. You can still build 32-bit Firefox on a 64-bit Windows installation.


The Mozilla build process requires many tools that are not pre-installed on most Windows systems. In addition to Visual Studio, install MozillaBuild - a software bundle that includes the required versions of bash, GNU make, Mercurial, and much more.

Firefox 61+ require Visual Studio 2017 Update 6 or newer to build.

Firefox 48 to 60 build with Visual Studio 2015. Visual Studio 2017 also works for building Firefox 58 or newer.

Firefox 37 through to 47 build with Visual Studio 2013 (VC12) and possibly Visual Studio 2015 (although Visual Studio 2015 may not build every revision).

Earlier versions of Firefox build with older versions of Visual Studio.

Installing the build prerequisites

Complete each of these steps otherwise, you may not be able to build successfully. There are notes on these software requirements below.

  1. Make sure your system is up-to-date through Windows Update.

  2. Install Visual Studio Community 2019 (free). Alternatively, you can also use a paid version of Visual Studio. Some additional components may be required to build Firefox, as noted in the “Visual Studio 2019” section above. Earlier versions of Visual Studio are not supported; the Firefox codebase relies on C++ features that are not supported in earlier releases.

  3. Optionally, in addition to VS2019, you may want to install Visual C++ 2008 Express (free) to compile some Python extensions used in the build system as Python 2.7.x for Windows is built with that compiler by default. Note, if you want to use “mach resource-usage”, “mach doctor”, “mach android-emulator”, or run talos tests locally, you should install it for building psutil.

  4. Download and install the MozillaBuild package, containing additional build tools. If you have Cygwin installed, read the note in the tips section. If you see a Windows error dialog giving you the option to re-install with the ‘correct settings’, after the MozillaBuild’s installer exits, choose the option and after that all should be well. More information about MozillaBuild and links to newer versions are available at


In some circumstances, the following problems can arise:

Antivirus performance

  • Windows Defender and some scanning antivirus products are known to have a major impact on build times. For example, if you have cloned mozilla-unified successfully but ./mach build fails, reporting a missing file, you are likely experiencing this problem. Our regression tests, for well-known security bugs, can include code samples that some antivirus software will identify as a threat, and will either quarantine or otherwise corrupt the files involved. To resolve this you will need to add your source and object directories (the mozilla-source and mozilla-build directories) to the exclusion list in Windows Defender or your antivirus software. If you are missing files, revert your source tree with the hg update -C command. Once this is done your next ./mach build should complete successfully.

Installing Visual Studio in a different language than the system can cause issues

  • For example, having Visual Studio in French when the system is in English causes the build to spew a lot of include errors and finishes with a link error.


Note: Mozilla 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.

MozillaBuild package

The MozillaBuild package contains other software prerequisites necessary for building Mozilla, including the MSYS build environment, Mercurial, CVS, Python, YASM, NSIS, and UPX, as well as optional but useful tools such as wget and emacs.

Download the current MozillaBuild package.

By default, the package installs to c:\mozilla-build and it is recommended to use the default path. Don’t use a path that contains spaces. The installer does not modify the Windows registry. Note that some binaries may require Visual C++ Redistributable package to run.


MozillaBuild command prompt expectation setting: Note that the “UNIX-like” environment provided by MozillaBuild is only really useful for building and committing to the Mozilla source. Most command line tools you would expect in a modern Linux distribution are not present, and those tools that are provided can be as much as a decade or so old (especially those provided by MSYS). It’s the old tools in particular that can cause problems since they often don’t behave as expected, are buggy, or don’t support command line arguments that have been taken for granted for years. For example, copying a source tree using cp -rf src1 src2 does not work correctly because of an old version of cp (it gives “cp: will not create hard link” errors for some files). In short, MozillaBuild supports essential developer interactions with the Mozilla code, but beyond that don’t be surprised if it trips you up in all sorts of exciting and unexpected ways.

Opening a MozillaBuild command prompt

After the prerequisites are installed, launch the start-shell.bat batch file using the Windows command prompt in the directory to which you installed MozillaBuild (c:\mozilla-build by default). This will launch an MSYS/BASH command prompt properly configured to build Firefox. All further commands should be executed in this command prompt window. (Note that this is not the same as what you get with the Windows CMD.EXE shell.)


Note: This is not the same as what you get with the Windows CMD.EXE shell.

Create a directory for the source

Note: You won’t be able to build the Firefox source code if it’s under a directory with spaces in the path such as “Documents and Settings”. You can pick any other location, such as a new directory c:/mozilla-source or c:/thunderbird-src. The build command prompt also tolerates “c:\” and “/c/”, but the former gives confusion in the Windows command prompt, and the latter is misinterpreted by some tools (at least MOZ_OBJDIR). The “C:/” syntax helps draw attention that the MozillaBuild command prompt is assumed from here on out since it provides configured environment and tools.

It’s a sensible idea to create a new shallow directory, like “c:/mozilla-source” dedicated solely to the code:

cd c:/; mkdir mozilla-source; cd mozilla-source

Keeping in mind the diagnostic hints below should you have issues. You are now ready to get the Firefox source and build.

Command prompt tips and caveats

  • To paste into this window, you must right-click on the window’s title bar, move your cursor to the “Edit” menu, and click “Paste”. You can also set “Quick Edit Mode” in the “Properties” menu and right-click the window to paste your selection.

  • If you have Cygwin installed, make sure that the MozillaBuild directories come before any Cygwin directories in the search path enhanced by start-shell-msvc2015.bat (use echo $PATH to see your search path).

  • In the MSYS / BASH shell started by start-shell-msvc2015.bat, UNIX-style forward slashes (/) are used as path separators instead of the Windows-style backward slashes (\). So if you want to change to the directory c:\mydir, in the MSYS shell to improve clarity, you would use cd /c/mydir ``though both ``c:\mydir and c:/mydir are supported.

  • The MSYS root directory is located in /c/mozilla-build/msys if you used the default installation directory. It’s a good idea not to build anything under this directory. Instead use something like /c/mydir.

Common problems, hints, and restrictions

  • Debugging Firefox on Windows FAQ: Tips on how to debug Mozilla on Windows.

  • Your installed MozillaBuild may be too old. The build system may assume you have new features and bugfixes that are only present in newer versions of MozillaBuild. Instructions for how to update MozillaBuild can be found here.

  • If the bootstrapping script fails, you can also try running hg clone followed by cd mozilla-unified; ./mach bootstrap yourself.

  • The build may fail if your machine is configured with the wrong architecture. If you want to build 64-bit Firefox, add the two lines below to your mozconfig file:

ac_add_options --target=x86_64-pc-mingw32
ac_add_options --host=x86_64-pc-mingw32
  • The build may fail if your PATH environment variable contains quotation marks(“). Quotes are not properly translated when passed down to MozillaBuild sub-shells and they are usually not needed so they can be removed.

  • The build may fail if you have a PYTHON environment variable set. It displays an error almost immediately that says “The system cannot find the file specified.” Typing “unset PYTHON” before running the Mozilla build tools in the same command shell should fix this. Make sure that PYTHON is unset, rather than set to an empty value.

  • The build may fail if you have Cygwin installed. Make sure that the MozillaBuild directories (/c/mozilla-build and subdirectories) come before any Cygwin directories in your PATH environment variable. If this does not help, remove the Cygwin directories from PATH, or try building on a clean PC with no Cygwin.

  • Building with versions of NSIS other than the version that comes with the latest supported version of MozillaBuild is not supported and will likely fail.

  • If you intend to distribute your build to others, set WIN32_REDIST_DIR=$VCINSTALLDIR\redist\x86\Microsoft.VC141.CRT in your mozconfig to get the Microsoft CRT DLLs packaged along with the application. Note the exact .CRT file may depend on your Visual Studio version.

  • The Microsoft Antimalware service can interfere with compilation, often manifesting as an error related to conftest.exe during build. To remedy this, add at your object directory at least to the exclusion settings.

  • Errors like “second C linkage of overloaded function ‘_interlockedbittestandset’ not allowed”, are encountered when intrin.h and windows.h are included together. Use a #define to redefine one instance of the function’s name.

  • Parallel builds (-jN) do not work with GNU makes on Windows. You should use the mozmake command included with current versions of MozillaBuild. Building with the mach command will always use the best available make command.

  • If you encounter a build failure like “ERROR: Cannot find makecab.exe”, try applying the patch from bug 1383578, i.e. change: SET PATH="%PATH%;!LLVMDIR!\bin" to SET "PATH=%PATH%;!LLVMDIR!\bin".

  • If you encounter a build failure with LINK: fatal error LNK1181: cannot open input file ..\..\..\..\..\security\nss3.lib, it may be related to your clone of mozilla-unified being located in the Users folder (possibly encrypted). Try moving it outside of the Users folder. The docs recommend C:\mozilla-source\mozilla-unified which should work.

  • If you encounter a build failure with ERROR: GetShortPathName returned a long path name..You need create a 8dot3name short name for the path which has space.For example : fsutil file setshortname “C:\Program Files (x86)” PROGRA~2. If you got “access denied”, try to restart your computer to safe mode and try again.