"Quantum CSS" is the new name for "Stylo", which is the new CSS rendering engine, a part of "Project Quantum" which is the project name to rewrite all of Firefox's internals, which will be called "Servo". I think there was a company memo to use the "replace a jet engine while the jet is flying" metaphor, but it's apt.
It's fascinating, but ultimately the win is for users of Firefox. Lin Clark:
It takes advantage of modern hardware, parallelizing the work across all of the cores in your machine. This means it can run up to 2 or 4 or even 18 times faster.
With any luck, CSS developers won't notice anything but the speed either.