The release of Firefox 57 'Quantum' last year included several major changes to the browser's structure, including a new CSS engine known as Quantum CSS (formerly called 'Stylo'). Simply put, Quantum CSS is the browser component that figures out what styles should be applied to what elements. Because modern sites often use thousands of lines of CSS with many overriding styles, the feature was designed to take advantage of multiple CPU cores, giving Firefox 57 a performance boost over previous versions.
Sadly, Quantum CSS did not arrive in Firefox 57 for Android. Mozilla initially delayed it to version 59, but that update was released yesterday without said feature. Thankfully, Quantum CSS has finally been enabled in the beta branch of Firefox for Android, starting with v60. I tried it out on my Pixel, but I didn't notice a major difference compared to v59. Maybe you'll have different results on your devices.
Firefox Beta 60 is still rolling out on the Play Store, but you can download it from APKMirror if you don't want to wait.
- Firefox Release Notes