Yes it does. Quantum is not new from scratch, it's (excellent) improvement and reorganization of largely the same code base. Some code and components are new, but that's rare all in all.
Quantum uses a new UI framework called Photon, new CSS engine and rendering architecture called Servo and the compositor has been completely overhauled to integrate Servo. So no, it is not largely based on a 6 year old codebase. Just Google it like I did if you don't believe me.
Photon is just the UX design guidelines that were overhauled with Quantum. Having said that, they were developed in close collaboration between the UX designers and UI engineers to ensure that it performs as best as it possibly can.
> new CSS engine and rendering architecture called Servo
Servo is an experimental engine developed by Mozilla Research. Its style system, dubbed Stylo, was adapted for use by Gecko.
> and the compositor has been completely overhauled to integrate Servo.
Gecko’s compositor was moved into its own process during Quantum, and Servo’s graphics subsystem, WebRender, is in the process of being gradually deployed on release, with several more cohorts pending.
Quantum does not use Servo. Servo is a research project, and _ideas_ from Servo get reimplemented into Firefox. As it stands, the main things that came from Servo are Stylo and WebRender. While they're two large things, there's still a _lot_ of Firefox that is old code. And a lot of things they still want to reintegrate.
Got some numbers on total line count and total modified line count? I know lines isn't a great proxy for complexity but don't know of a better one available in this context.
I don't think the number of lines changed, removed and added it going to be all that useful. If anything, it will likely lead to erroneous snap judgements.
