https://www.phoronix.com/scan.php?page=news_item&px=Firefox-Clang-LTO-All-Platforms
"Firefox nightly builds are now built with the LLVM Clang compiler on all major platforms and the Linux build in particular is also now utilizing PGO optimizations too. Faster Firefox is coming thanks to this compiler work. All of Mozilla's tier-one platforms are now building the newest Firefox browser code under the Clang compiler and having LTO (Link Time Optimizations <https://www.phoronix.com/vr.php?view=17785>) enabled. That includes Linux, Mac, Android, Windows across ARM / AArch64 / x86 relying upon this open-source compiler. For now only the Linux builds also have PGO (Profile Guided Optimizations <https://www.phoronix.com/vr.php?view=26589>) enabled. With the compiler re-tooling on Linux they are seeing about 5% better performance and upwards of 18% in select tests compared to their previous GCC (6.4-based) compiler with PGO. The current Clang stack they are using is based on LLVM6 while when they transition to the upcoming LLVM/Clang 7.0 they expect 2~5% better performance on top of that. Mozilla developers did try GCC LTO as well as switching to the newest GCC8 release, but ran into problems. These new release binaries should go into effect for the Firefox 64 release."
