Firefox 64 built with GCC and Clang
> Clang built Firefox is claimed to outperform GCC, but it is hard to get actual numbers. Firefox builds switched from GCC 6 builds (GCC 6 was released in 2016) with profile guided optimization (PGO) to Clang 7 builds (latest release) which in addition enable link time optimization (LTO). Link-time optimization can have important performance and code size impact.
Plus all the difficulties encountered trying to enable all the options.