This is not correct. Servo predated Quantum. Quantum was never abandoned, it was completed and integrated into Firefox. Quoting from their page on Quantum:
>Firefox Quantum was a revolution in Firefox development. In 2017, we created a new, lightning fast browser that constantly improves. Firefox Quantum is the Firefox Browser.
Quantum took modernizations that were started as part of Servo but integrated them into Gecko, where they continue to live today. They include Stylo as you noted, but also WebRender, Quantum DOM, Quantum Compositor, Quantum Flow, as well as a host of rust libraries and tooling.
Firefox’s responsiveness, GPU acceleration, and memory-safety improvements are directly attributable to Servo's research being folded in.
So it is in fact getting the benefits I stated. In fact it demonstrates that Servo succeeded after all: the fruits of it were harvested, brought into Firefox and are a key demonstration of Mozilla punching above its weight with smart investments allowing them to be more efficient than Google.
Edit: The reply below is largely unresponsive, not acknowledging factual inaccuracies, and repeating points that have already been addressed. As I already explained, Firefox benefits from Servo to this day. The end of its support is regrettable but not a scandal nor even a demonstration of inefficient use of resources. Its benefits were rolled into the core browser which is maintained at a fraction of the cost of its competition.
There was a time when Mozilla worked on what is being now pushed forward under the Servo umbrella. 2017 was 8 years ago. Then in 2020 Mozilla laid off 250 employees, including pretty much all of the developers of Servo/Quantum and their security team.
And now Mozilla is sitting on that cool $1.3 B, burning $220-500 M/year on "software development" without telling anyone what it actually is doing with that money, and putting $0 into Servo.
Firefox contains not-so-much Rust and Servo would have been the road there.
Mozilla is specifically not developing Servo, and thus mostly not getting the benefits you state. (The major exception being Stylo.)