I might be mistaken, but I thought Stylo (the CSS part of Servo) was shipped as part of Firefox in 2017.
Yes, that's true.
But Servo, to the best of my knowledge, was* an R&D project to develop an new experimental engine. The parts lifted into Firefox are now presumably maintained and developed as a part of Firefox proper.
Don't get me wrong, I was disappointed when i heard that the Servo team was laid off. We are probably missing out on new exciting things that could have come out of it. I just wanted to clarify that cutting Servo did not, as far as I know, mean that any current part of Firefox is now unmaintained or undeveloped.
* Servo is still alive and developed outside of Mozilla, I don't know if's doing well or not.
Just to clarify, Servo was never a part of Firefox. Maybe that's not what you meant, but I've seen it confused with Gecko before which is the actual browser engine used by Firefox.
And as much as I am puzzled by some of the design choices lately - for the android version of Firefox especially - I think that it would be a loss in more ways than one to lose the last (?) real alternative to the Chromium-based family of browsers.