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+Supported Build Hosts and Targets
+=================================
+
+ ..  role:: strikethrough
+
+There are three tiers of supported Firefox build hosts and targets.
+These tiers represent the shared engineering priorities of the Mozilla project.
+
+The "build host" is the machine that is performing the build of Firefox, and
+the "build target" is the machine that will run the built Firefox application.
+For example, if you were building Firefox for Android on your Linux computer, then the
+Linux computer would be the "build host".
+
+.. note::
+
+   Sheriffs are in charge of monitoring the tree. Their definition for tiers
+   is for automation jobs, which tells a developer what is expected of them when
+   they land code. This document is about the tiers of supported build hosts and targets,
+   which tells a person compiling/using Firefox what they can expect from Mozilla.
+   See the `job tier definition <https://wiki.mozilla.org/Sheriffing/Job_Visibility_Policy#Overview_of_the_Job_Visibility_Tiers>`__ for more information.
+
+
+.. _build_hosts:
+
+Supported Build Hosts
+---------------------
+
+While we want to help users resolve build-related issues on their systems, we
+are unable to help resolve build system issues on all possible operating
+systems and versions.
+
+.. _tier_1_hosts:
+
+Tier-1 Hosts and Toolchains
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Support is available for the following **host operating systems** and versions
+when building for a :ref:`Tier-1 Firefox build target<tier_1_targets>`, including
+cross-compilation where available:
+
+* Ubuntu Linux x86_64
+    * Current stable release
+    * Previous stable release
+    * Current LTS release
+* Debian Linux x86_64
+    * Current stable release
+    * Current testing release
+* Fedora Linux x86_64
+    * Current stable release
+    * Previous stable release
+* macOS Intel and M1
+    * Current major macOS release
+    * Previous major macOS release
+* Windows x86_64
+    * Windows 10 with MozillaBuild Environment
+    * Windows 11 with MozillaBuild Environment
+
+Tier-2 Hosts and Toolchains
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In addition to the limitations outlined above in the Tier-1 list, our ability
+to provide assistance with build issues using/targeting Tier-2
+hosts/targets/compilers is not unbounded.
+
+While we will endeavour to make a best effort to help resolve issues, you may
+be referred to the relevant community maintainers for further support.
+
+The Tier-2 hosts are:
+
+* Other Linux x86_64 distributions and/or versions
+* Older macOS versions
+* Older Windows x86_64 versions
+
+Tier-3 Hosts and Toolchains
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+We cannot provide any guarantees of assistance in resolving build issues using
+or targeting Tier-3 platforms.
+
+
+Supported Build Targets
+-----------------------
+
+.. _tier_1_targets:
+
+Tier-1 Targets
+^^^^^^^^^^^^^^
+
+The term **"Tier-1 platform"** refers to those platforms - CPU
+architectures and operating systems - that are the primary focus of
+Firefox development efforts. Tier-1 platforms are fully supported by
+Mozilla's `continuous integration processes <https://treeherder.mozilla.org/>`__ and the
+:ref:`Pushing to Try`. Any proposed change to Firefox on these
+platforms that results in build failures, test failures, performance
+regressions or other major problems **will be reverted immediately**.
+
+
+The **Tier-1 Firefox platforms** and their supported compilers are:
+
+-  Android on Linux x86, x86-64, ARMv7 and ARMv8-A (clang)
+-  Linux/x86 and x86-64 (gcc and clang)
+-  macOS 10.12 and later on x86-64 and AArch64 (clang)
+-  Windows/x86, x86-64 and AArch64 (clang-cl)
+
+Prior to Firefox 63, Windows/x86 and Windows/x86-64 relied on the MSVC
+compiler; from **Firefox 63 onward MSVC is not supported**. Older 32-bit
+x86 CPUs without SSE2 instructions such as the Pentium III and Athlon XP
+are also **not considered Tier-1 platforms, and are not supported**.
+Note also that while Windows/x86 and ARM/AArch64 are supported *as build
+targets*, it is not possible to build Firefox *on* Windows/x86 or
+Windows/AArch64 systems.
+
+Tier-2 Targets
+^^^^^^^^^^^^^^
+
+**Tier-2 platforms** are actively maintained by the Mozilla community,
+though with less rigorous requirements. Proposed changes resulting in
+breakage or regressions limited to these platforms **may not immediately
+result in reversion**. However, developers who break these platforms are
+expected to work with platform maintainers to fix problems, and **may be
+required to revert their changes** if a fix cannot be found.
+
+The **Tier-2 Firefox platforms** and their supported compilers are:
+
+-  Linux/AArch64 (clang)
+-  Windows/x86 (mingw-clang) - maintained by Tom Ritter and Jacek Caban
+   -
+
+   -  *Note that some features of this platform are disabled, as they
+      require MS COM or the w32api project doesn't expose the necessary
+      Windows APIs.*
+
+Tier-3 Targets
+^^^^^^^^^^^^^^
+
+**Tier-3 platforms** have a maintainer or community which attempt to
+keep the platform working. These platforms are **not supported by our
+continuous integration processes**, and **Mozilla does not routinely
+test on these platforms**, nor do we block further development on the
+outcomes of those tests.
+
+At any given time a Firefox built from mozilla-central for these
+platforms may or may not work correctly or build at all.
+
+**Tier-3 Firefox platforms** include:
+
+-  Linux on various CPU architectures including ARM variants not listed
+   above, PowerPC, and x86 CPUs without SSE2 support - maintained by
+   various Linux distributions
+-  FreeBSD/x86, x86-64, Aarch64 (clang) - `maintained by gecko@FreeBSD.org <https://www.freshports.org/www/firefox/>`__
+-  OpenBSD/x86, x86-64 (clang) - maintained by Landry Breuil
+-  NetBSD/x86-64 (gcc) - maintained by David Laight
+-  Solaris/x86-64, sparc64 (gcc) - maintained by Petr Sumbera
+-  :strikethrough:`Windows/x86-64 (mingw-gcc)` - Unsupported due to
+   requirements for clang-bindgen
+
+If you're filing a bug against Firefox on a Tier-3 platform (or any
+combination of OS, CPU and compiler not listed above) please bear in
+mind that Mozilla developers do not reliably have access to non-Tier-1
+platforms or build environments. To be actionable bug reports against
+non-Tier-1 platforms should include as much information as possible to
+help the owner of the bug determine the cause of the problem and the
+proper solution. If you can provide a patch, a regression range or
+assist in verifying that the developer's patches work for your platform,
+that would help a lot towards getting your bugs fixed and checked into
+the tree.