"Build Metrics" is a catch-all term for performance measures that are generated by the Firefox build system and tracked by Perfherder.
Number of static constructors found by the compiler in the Firefox C++ codebase. Lower is better. Static constructors are undesirable because their initialization imposes an unavoidable time penalty every time Firefox is started.
Size in bytes of the Firefox installer. Lower is better here, especially on space-restricted platforms like Android.
Amount of time it takes to build Firefox in automation on a specific platform / configuration. Lower is better.