1.1 Install Visual Studio Build Tools¶

Download and install the Build Tools for Visual Studio 2022. If you have a full install of Visual Studio (Community/Professional/Enterprise), that is also supported. Ensure you’ve checked the following items for installation:

• In the Workloads tab:

  • Desktop development with C++.

• In the Individual components tab:

  • MSVC v143 - VS 2022 C++ x64/x86 build tools.

  • Windows 10 SDK (at least 10.0.19041.0).

  • C++ ATL for v143 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).

cd c:/
mkdir mozilla-source
cd mozilla-source
wget https://hg.mozilla.org/mozilla-central/raw-file/default/python/mozboot/bin/bootstrap.py
python3 bootstrap.py

When running bootstrap.py 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.

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 bootstrap.py --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 bootstrap.py 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.

cd c:/mozilla-source/mozilla-unified
./mach build
./mach run

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.

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.