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.
Mozilla can't help it but be their own worst enemy. Ladybird may well never have happened if Mozilla just had kept working on Servo, and Ladybird is most definitely going to out compete Firefox when it reaches maturity, as Mozilla keeps on burning bridges with open source enthusiasts.
The problem with Mozilla is not just technical but cultural. The organization has been infected with managers. The managers want to keep their jobs more than they want Firefox to succeed. Clearly the solution is for the managers to fire themselves and allow the developers to run the show, but that was not going to happen.
Ladybird, by contrast, is a developer-lead open source project that has no such constraints. They also don't have a product yet but I'm sure the picture will be radically different in a few years.
To add to the other replies, Firefox was explicitly never going to consume all of Servo. It was always meant to be a test bed project where sub-projects could be migrated to Firefox. I suspect that the long term intent might have been for Servo to get to a point where it could become Firefox, but that wasn't the stated plan.
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.
Yes, during the layoff of August 2020
Mozilla laid off the full Servo team, but never publicly announced this afaik. Wikipedia includes it here: https://en.wikipedia.org/wiki/Firefox#cite_ref-120
Mozilla can't help it but be their own worst enemy. Ladybird may well never have happened if Mozilla just had kept working on Servo, and Ladybird is most definitely going to out compete Firefox when it reaches maturity, as Mozilla keeps on burning bridges with open source enthusiasts.
The problem with Mozilla is not just technical but cultural. The organization has been infected with managers. The managers want to keep their jobs more than they want Firefox to succeed. Clearly the solution is for the managers to fire themselves and allow the developers to run the show, but that was not going to happen.
Ladybird, by contrast, is a developer-lead open source project that has no such constraints. They also don't have a product yet but I'm sure the picture will be radically different in a few years.
Conway's law in action.
To add to the other replies, Firefox was explicitly never going to consume all of Servo. It was always meant to be a test bed project where sub-projects could be migrated to Firefox. I suspect that the long term intent might have been for Servo to get to a point where it could become Firefox, but that wasn't the stated plan.
I think they implemented parts of it into their Gecko engine. But they laid of all the Servo development team in like 2020 I believe.
Only recently when it moved over to the Linux Foundation has Servo started being worked on again