Comment by dralley

Firefox incorporated parts of the Servo effort which were able to reach maturity. Stylo (Firefox's current CSS engine) and Webrender (the rendering engine) and a few other small components came from the Servo project.

Most other parts of Servo were not mature enough to integrate at the time Mozilla decided to end support for the project and didn't look like they would be mature enough any time soon. The DOM engine for example was in the early stages of being completely rewritten at the time because the original version had an architecture that made supporting the entire breadth of web standards challenging.

Keep in mind that you can continue adding Rust to Firefox without replacing whole components. It's not like Mozilla abandoned the idea of using more Rust in Firefox just because they stopped trying to rewrite whole components from the ground up.