Configuring Build Options
=========================

This document details how to configure Firefox builds.
Most of the time a ``mozconfig`` file is not required. The default
options are the most well-supported, so it is preferable to add as few
options as possible. Please read the following directions carefully
before building, and follow them in order. Skipping any step may cause
the build to fail, or the built software to be unusable. Build options,
including options not usable from the command-line, may appear in
"``confvars.sh``" files in the source tree.


Using a ``mozconfig`` configuration file
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The choice of which Mozilla project to build and other configuration
options can be configured in a ``mozconfig`` file. (It is possible to
manually call ``configure`` with command-line options, but this is not
recommended). The ``mozconfig`` file should be in your source directory
(that is, ``/mozilla-central/mozconfig``).

Create a blank ``mozconfig`` file:

.. code:: bash

    echo "# My first mozilla config" > mozconfig

If your mozconfig isn't in your source directory, you can also use the
``MOZCONFIG`` environment variable to specify the path to your
``mozconfig``. The path you specify **must** be an **absolute** path or
else ``client.mk`` will not find it. This is useful if you choose to
have multiple ``mozconfig`` files for different projects or
configurations (see below for a full example). Note that in the
``export`` example below the filename was not ``mozconfig``. Regardless
of the name of the actual file you use, we refer to this file as the
``mozconfig`` file in the examples below.

Setting the ``mozconfig`` path:

.. code:: bash

   export MOZCONFIG=$HOME/mozilla/mozconfig-firefox

.. note::

   Calling the file ``.mozconfig`` (with a leading dot) is also
   supported, but this is not recommended because it may make the file
   harder to find. This will also help when troubleshooting because
   people will want to know which build options you have selected and
   will assume that you have put them in your ``mozconfig`` file.


``mozconfig`` contains two types of options:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

-  Options prefixed with ``mk_add_options`` are passed to
   ``client.mk``. The most important of these is ``MOZ_OBJDIR``, which
   controls where your project gets built (also known as the object
   directory).
-  Options prefixed with ``ac_add_options`` are passed to ``configure``,
   and affect the build process.


Building with an objdir
~~~~~~~~~~~~~~~~~~~~~~~

This means that the source code and object files are not intermingled in
your directory system and you can build multiple projects (e.g.,
Firefox and Thunderbird) from the same source tree. If you do not
specify a ``MOZ_OBJDIR``, it will be automatically set to
``@TOPSRCDIR@/obj-@CONFIG_GUESS@``.

If you need to re-run ``configure``, the easiest way to do it is using
``./mach configure``; running ``configure`` manually is strongly
discouraged.

Adding the following line to your ``mozconfig`` allows you to change the
objdir:

.. code:: bash

   mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/obj-@CONFIG_GUESS@

It is a good idea to have your objdir name start with ``obj`` so that
the VCS ignores it.

Sometimes it can be useful to build multiple versions of the source
(such as with and without diagnostic asserts). To avoid the time it
takes to do a full rebuild, you can create multiple ``mozconfig`` files
which specify different objdirs. For example, a ``mozconfig-dbg``:

.. code:: bash

   mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/obj-ff-dbg
   ac_add_options --enable-debug

and a ``mozconfig-rel-opt``:

.. code:: bash

   mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/obj-ff-rel-opt
   ac_add_options --disable-debug
   ac_add_options --enable-optimize

allow for building both versions by specifying the configuration via
the ``MOZCONFIG`` environment variable:

.. code:: bash

   $ env MOZCONFIG=/path/to/mozconfig-dbg ./mach build
   $ env MOZCONFIG=/path/to/mozconfig-rel-opt ./mach build

Don't forget to set the ``MOZCONFIG`` environment variable for the
``mach run`` command as well.

Be aware that changing your ``mozconfig`` will require the configure
process to be rerun and therefore the build will take considerably
longer, so if you find yourself changing the same options regularly, it
may be worth having a separate ``mozconfig`` for each. The main downside
of this is that each objdir will take up a significant amount of space
on disk.


Parallel compilation
~~~~~~~~~~~~~~~~~~~~

.. note::

   The build system automatically makes an intelligent guess
   for how many CPU cores to use when building. The option below is
   typically not needed.

Most modern systems have multiple cores or CPUs, and they can be
optionally used concurrently to make the build faster. The ``-j`` flag
controls how many parallel builds will run concurrently. You will see
(diminishing) returns up to a value approximately 1.5× to 2.0× the
number of cores on your system.

.. code:: bash

   mk_add_options MOZ_PARALLEL_BUILD=4

If your machine is overheating, you might want to try a lower value.


Choose a project
~~~~~~~~~~~~~~~~

The ``--enable-project=project`` flag is used to select a project to
build. Firefox is the default.

Choose one of the following options to add to your ``mozconfig`` file:

Browser (Firefox)
   .. code::

      ac_add_options --enable-project=browser

   .. note::

      This is the default

Mail (Thunderbird)
   .. code::

      ac_add_options --enable-project=comm/mail

Mozilla Suite (SeaMonkey)
   .. code::

      ac_add_options --enable-project=suite

Calendar (Lightning Extension, uses Thunderbird)
   .. code::

      ac_add_options --enable-project=comm/mail
      ac_add_options --enable-calendar


Selecting build options
~~~~~~~~~~~~~~~~~~~~~~~

The build options you choose depends on what project you are
building and what you will be using the build for. If you want to use
the build regularly, you will want a release build without extra
debugging information; if you are a developer who wants to hack the
source code, you probably want a non-optimized build with extra
debugging macros.

There are many options recognized by the configure script which are
special-purpose options intended for embedders or other special
situations, and should not be used to build the full suite/XUL
projects. The full list of options can be obtained by running
``./mach configure -- --help``.

.. warning::

   Do not use a configure option unless you know what it does.
   The default values are usually the right ones. Each additional option
   you add to your ``mozconfig`` file reduces the chance that your build
   will compile and run correctly.

The following build options are very common:

sccache
^^^^^^^

`SCCache <https://github.com/mozilla/sccache>`__ allows speeding up subsequent
C / C++ builds by caching compilation results. Unlike
`ccache <https://ccache.dev>`__, it also allows caching Rust artifacts, and
supports `distributed compilation
<https://github.com/mozilla/sccache/blob/master/docs/DistributedQuickstart.md>`__.

In order to enable ``sccache`` for Firefox builds, you can use
``ac_add_options --with-ccache=sccache``.

From version 0.7.4, sccache local builds are using the ``preprocessor cache mode``
by default. With a hot cache, it decreases the build time by a factor of 2 to 3
compared the previous method. This feature works like the `direct mode in ccache
<https://ccache.dev/manual/3.7.9.html#_the_direct_mode>`__,
using a similar way to handle caching and dependencies.

   .. note::

      When using sccache, because of the operation on the files and storage,
      the initial build of Firefox will be slower.

Optimization
^^^^^^^^^^^^

``ac_add_options --enable-optimize``
   Enables the default compiler optimization options

   .. note::

      This is enabled by default

``ac_add_options --enable-optimize=-O2``
   Chooses particular compiler optimization options. In most cases, this
   will not give the desired results, unless you know the Mozilla
   codebase very well; note, however, that if you are building with the
   Microsoft compilers, you probably **do** want this as ``-O1`` will
   optimize for size, unlike GCC.
``ac_add_options --enable-debug``
   Enables assertions in C++ and JavaScript, plus other debug-only code.
   This can significantly slow a build, but it is invaluable when
   writing patches. **People developing patches (especially in C++)
   should generally use this option.**
``ac_add_options --disable-optimize``
   Disables compiler optimization. This makes it much easier to step
   through code in a debugger.
``ac_add_options --enable-release``
   Enables more conservative, release engineering-oriented options. This may
   slow down builds. This also turns on full optimizations for Rust. Note this
   is the default when building release/beta/esr.
``ac_add_options --enable-debug-js-modules``
   Enable only JavaScript assertions. This is useful when working
   locally on JavaScript-powered components like the DevTools. This will
   help catch any errors introduced into the JS code, with less of a
   performance impact compared to the ``--enable-debug`` option.
``export RUSTC_OPT_LEVEL=2``
   Enable full optimizations for Rust code.

You can make an optimized build with debugging symbols. See :ref:`Building
with Debug Symbols <Building with Debug Symbols>`.

Building as Beta or Release
^^^^^^^^^^^^^^^^^^^^^^^^^^^

``ac_add_options --as-milestone=release``
   This makes it easy to build nightly with a release or beta configuration to
   test the different ifdef behaviors. To do a full beta simulation see
   `Sheriffing/How To/Beta simulations <https://wiki.mozilla.org/Sheriffing/How_To/Beta_simulations>`__.

   - ``early-beta``
   - ``late-beta``
   - ``release``

Tests
^^^^^

``ac_add_options --disable-tests``
   By default, many auxiliary test programs are built, which can
   help debug and patch the mozilla source. Disabling these tests can
   speed build time and reduce disk space considerably. Developers
   should generally not use this option.

Localization
^^^^^^^^^^^^

``mk_add_options MOZ_CO_LOCALES=ISOcode``
   TBD.
``ac_add_options --enable-ui-locale=ISOcode``
   TBD.
``ac_add_options --with-l10n-base=/path/to/base/dir``
   TBD.

Other Options
^^^^^^^^^^^^^

``mk_add_options AUTOCLOBBER=1``
   If a clobber would be required before a build, this will cause mach
   to clobber and continue with the build instead of asking the user to
   manually clobber and exiting.

``ac_add_options --enable-warnings-as-errors``
   This makes compiler warnings into errors which fail the build. This
   can be useful since certain warnings coincide with reviewbot lints
   which must be fixed before merging.

.. _Example_.mozconfig_Files:

Example ``mozconfig`` Files
~~~~~~~~~~~~~~~~~~~~~~~~~~~

Mozilla's official builds use mozconfig files from the appropriate
directory within each repository.

.. warning::

   These ``mozconfig`` files are taken from production builds
   and are provided as examples only. It is recommended to use the default
   build options, and only change the properties from the list above as
   needed. The production builds aren't really appropriate for local
   builds."

-  .. rubric:: Firefox, `Debugging Build (macOS
      64bits) <https://github.com/mozilla-firefox/firefox/blob/main/browser/config/mozconfigs/macosx64/debug>`__
      :name: Firefox.2C_Default_Release_Configuration

Building multiple projects from the same source tree
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It is possible to build multiple projects from the same source tree,
as long as you `use a different objdir <#Building_with_an_Objdir>`__ for
each project.

You need to create multiple ``mozconfig`` files.

As an example, the following steps can be used to build Firefox and
Thunderbird. You should first create three ``mozconfig`` files.

``mozconfig-common``:

.. code::

   # add common options here, such as making an optimized release build
   mk_add_options MOZ_PARALLEL_BUILD=4
   ac_add_options --enable-optimize --disable-debug

``mozconfig-firefox``:

.. code::

   # include the common mozconfig
   . ./mozconfig-common

   # Build Firefox
   mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/obj-firefox
   ac_add_options --enable-project=browser

``mozconfig-thunderbird``:

.. code::

   # include the common mozconfig
   . ./mozconfig-common

   # Build Thunderbird
   mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/obj-thunderbird
   ac_add_options --enable-project=comm/mail

To build Firefox, run the following commands:

.. code::

   export MOZCONFIG=/path/to/mozilla/mozconfig-firefox
   ./mach build

To build Thunderbird, run the following commands:

.. code::

   export MOZCONFIG=/path/to/mozilla/mozconfig-thunderbird
   ./mach build

Using mozconfigwrapper
^^^^^^^^^^^^^^^^^^^^^^

Mozconfigwrapper is similar to using multiple mozconfig files except
that it abstracts and hides them so you don't have to worry about where
they live or which ones you've created. It also saves you from having to
export the MOZCONFIG variable each time. For information on installing
and configuring mozconfigwrapper, see
https://github.com/ahal/mozconfigwrapper.

Changing build options per directory -- the moz.build hook
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Adding a statement like the following to a ``mozconfig`` file:

.. code::

    ac_add_options MOZ_BUILD_HOOK=/absolute/path/to/file/buildhook.py

will cause this file to be appended to each ``moz.build`` file of the tree. It
is recommended to place this file outside of the tree, so that it is not wiped
when cleaning up the tree via ``git clean``.

This hook file is written in the same subset of Python as ``moz.build`` files.

Possibilities are endless, but this is particularly useful to tweak compiler
options for selected directories, such as disabling optimizations to have a
better debugging experience without having the entire browser being very slow:

.. code:: python

    nonopt = (
        "dom/media/",
        "media/ffvpx/",
        "media/libcubeb/",
    )
    if RELATIVEDIR.startswith(nonopt):
        COMPILE_FLAGS["OPTIMIZE"] = []

will make all compilation units at or under those three paths have no
optimization.

Another useful thing to set per directory is ``FILES_PER_UNIFIED_FILE=1`` to
disable :ref:`unified builds<unified_builds>`.
