Most of those benchmarks are (directly or indirectly) benchmarks of the JavaScript engine. The few that aren't are instead benchmarking the <canvas> implementat

Most of those benchmarks are (directly or indirectly) benchmarks of the JavaScript engine. The few that aren't are instead benchmarking the <canvas> implementation or the compositor. The main speedups in Firefox 57 are in styling and the DOM which are more of a "feels faster" thing than something you can easily benchmark in real-world scenarios other than "how long does this page take to load?"