Rust is being used to implement an experimental new browser engine with a focus on concurrency in order to better take advantage of highly-parallel and power-limited hardware. It's known as Servo,[1] and it's still strictly in research mode; there are no concrete plans to integrate anything into Firefox as of yet. Even if there were, Rust itself won't be at 1.0 until sometime next year, and it would be quite a while beyond that before Servo had any semblance of feature parity with Gecko.
