I think they’re playing a physics/geeky joke on “Quantum Leap”. Means the browser speed between two versions (56, 57) is far more than any other previous two version.
And, basically they’re fuzzing a slowly blend-in approach, which begin with adding rust from 48.0, and nowadays many part of the system is replaced by rust code.
The leap not only occurs with the UI refactoring or the strip of old extension API, but also with the js rendering engine and CSS parser. Which becomes quite stable and default since this (57.0) version.
However, upstream doesn’t care much about a shared-lib system, since they have to maintain an universal functional (automatic) packaging system, they tends to focus on internal bundle of a lot of dpendencies, and sometimes, the linking part is hard-coded that leads to broken compilation of shared linking.
So, we currently still disable the new CSS parser, stylo, for this release.
IIRC, there is a testing project to utilize full servo as browser engine, but the plan is to fully replace Firefox with the new core in the near future (59.0?).