Firefox was special in that Mozilla created Rust to build Servo and then backported parts of Servo to Firefox and ultimately stopped building Servo.

Thankfully Servo has picked up speed again and if one wants a Rust based browser engine what better choice than the one the language was built to enable?

https://servo.org/

As a Servo contributor, I am aware of Servo :)

But I'm also cheering along Ladybird's progress. There's definitely room for more than one project in the space. And IMO the more browsers being built in Rust in the better.