Not only is Firefox using it for their CSS engine but Mozilla created Rust to build Servo and sadly only the CSS engine and maybe some other parts is what they kept around when they offloaded Rust.

“the Rust ecosystem around browsers is growing” – in the beginning pretty much 100% of the ecosystem around Rust was browser oriented

Thankfully Servo is picking up speed again and is a great project to help support with some donations etc: https://servo.org/