You might have read that Mozilla recently switched Windows builds to clang-cl. More recently, those Windows builds have seen both PGO (Profile-Guided Optimization) and LTO (Link-Time Optimization) enabled. As of next nightly (as of writing, obviously), all tier-1 platforms are now built with clang with LTO enabled. Yes, this means Linux, Mac and Android arm, aarch64 and x86. Linux builds also have