You might want to give Nightly a try (upcoming Firefox 56) which improved a lot thanks to Quantom Flow project. I updated from Firefox Dev Edition (FF 55) to Nightly and the boast was obvious (there's also been a good boast recently in FF 54 thanks to multi-e10s).

e.g. see the SpeedometerV2 benchmark differences at https://ehsanakhgari.org/blog/2017-06-23/quantum-flow-engine... So there is progress and thanks to Quantum the best is hopefully yet to come with the integration of Servo components