Servo continues to be developed and Quantum is the project aiming to be a replacement to Gecko within Firefox. The hope is to transition to Quantum in mainline Firefox by the end of next year. The migration to the new engine is expected to be gradual over 2017 as Gecko gets gutted and replaced by Servo/Quantum code.
Quantum is Mozilla's project to build the next-generation web engine for Firefox users, building on the Gecko engine as a solid foundation. Quantum will leverage the fearless concurrency of Rust and high-performance components of Servo to bring more parallelization and GPU offloading to Firefox.Limited details so far via the Quantum Wiki page. Some early work on Quantum is focused on Quantum CSS )Stylo), Quantum Render for next-gen renderer making use of GPU rendering (WebRender), Quantum Compositor as the GPU code that will run in its own process, Quantum DOM, and Quantum Flow.
As the project evolves we'll be adopting more of Servo's components, and leveraging the research that helped build them, to make Firefox faster and more reliable than ever. Servo is a research project and doesn’t yet have all the functionality users expect from a full-featured web browser. By taking an incremental approach and merging proven components from Servo to Gecko as they are ready, users won't have to wait long to start seeing dramatic improvements to Firefox stability and performance. We're going to ship major improvements in 2017, and we'll iterate from there.
There is also this recent presentation on Quantum/Servo: