They already are. Mozilla Firefox uses a lot of multi threading and offloads a lot of the rendering to the GPU. So does Chrome. That's a big reason why Mozilla invented Rust: to be able to do this without dealing with a lot of concurrency, security, and stability bugs.

The creator of Rust has been very clear that Mozilla did not invent Rust (https://twitter.com/graydon_pub/status/1492634815748739077)

I stand corrected.

But lets just say they were extremely early adopters and single-handedly propelled Rust into the mainstream by essentially bankrolling Rust development for several years until they finally spun it off into a separate foundation a few years ago.

I doubt it as it has a "proprietary rendering engine", but Servo (https://servo.org/) has been doing a similar thing and from what I remember, it is basically a safe multi-threaded rendering engine.