If you’re on Firefox Nightly, you are able to enable their new CSS engine with an about:config flag, called layout.css.servo.enabled. For a few years now, Mozilla has been working on a separate rendering engine, aided by Samsung, which was called Servo. Browsers are very single-threaded, so there was a lot of room for improvement, especially on devices that can afford more cores than per-core performance, like mobile. It is also more secure, as its programming language, Rust, is more strict with data accesses than C/C++, which is also great for a web browser.
Eventually, Mozilla decided to, instead of replacing Gecko, replace chunks of it with tech derived from Servo. Up to now, it’s been mostly security-related components, like the parsing of untrusted media headers. This one is about speed. I'm curious to see how it feels to our readers. I know that, personally, going from Firefox 54 to Firefox 55 was a significant difference, although that was due to other changes.
If you’re interested, download Firefox Nightly. I mean, it’s free.