There are three tiers of supported Mozilla build targets at this time. These tiers represent the shared priorities of the Mozilla project.
The term "Tier-1 platform" refers to those platforms that are the primary focus for development. Build errors, performance regressions and other major problems on these platforms are considered showstoppers and will cause that change to be immediately reverted. These are also the platforms that show up on the Firefox continuous integration, and run on the Mozilla try-server. Tier-1 platforms are:
- Android Linux/x86, ARMv7 and ARMv8-A (clang)
- Linux/x86 and x86-64 (gcc and clang)
- OS X/x86-64 (clang)
- Windows/x86 and Windows/x86-64 (msvc) (before Firefox 63)
- Windows/x86 and Windows/x86-64 (clang-cl) (as of Firefox 63)
Older 32 bit x86 CPUs without SSE2 instructions such as the Pentium III and Athlon XP are not supported by official builds. They are not considered to be part of Tier-1.
Tier-2 platforms are those platforms that the Mozilla community actively maintains. Breakage or regressions in these platforms does not immediately close the tree; developers who break these platforms are expected to work with platform maintainers to fix problems, and may be required to back out their changes if a fix cannot be found:
- Windows/x86 and Windows/x86-64 (msvc) (as of Firefox 63)
- Windows/x86 (mingw gcc) — maintained by Tom Ritter and Jacek Caban — some features are disabled because they require MS COM or the w32api project doesn't expose the necessary Windows APIs
Tier-3 platforms have a maintainer or community which attempt to keep the platform working. These platforms may or may not work at any time, and often have little test coverage:
- Linux on various CPU architectures including ARM, PPC and x86 CPUs without SSE2 support — maintained by various Linux distributions
- FreeBSD/x86, x86-64, aarch64 (clang) — maintained by Jan Beich
- 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
- Windows/x86-64 (mingw gcc) — maintained by Jacek Caban — some features are disabled because they require MS COM or the w32api project doesn't expose the necessary Windows APIs
Most Mozilla developers do not have access to non-tier-1 platforms so any bug reports against non-tier-1 platforms should be overflowing with information to help the owner of the bug determine the cause of the problem and the proper solution. If you can provide a patch and/or verify that the developer's patches work for your platform, that would help a lot towards getting your bugs fixed and checked into the tree.
This document previously referred to "supported build configurations", but since building on Android is unsupported (and as of mid-2015 you can't build Firefox on Win/x86 at all) it was changed to "supported build targets".