'On a course' implies that there's some sort of definite plan and resources delegated to the task. Which isn't true of Firefox and Rust, as far as I know.

While it may not be an explicit goal, everything I’ve read implies that, for example the heading in this wiki on oxidization: https://wiki.mozilla.org/Oxidation

> Moving forward, the goal of Oxidation is to make it easier and more pleasant to use Rust in Firefox, and correspondingly to increase the amount of Rust code in Firefox.

And then there is the Rust page on Mozilla’s site:

> Mozilla also utilizes Rust in many of its core initiatives including Servo and key parts of Firefox.

https://research.mozilla.org/rust/

Where “key parts” is a link to the project quantum announcement: https://medium.com/mozilla-tech/a-quantum-leap-for-the-web-a...

> Initially, Quantum will share a couple of components with Servo, but as the projects evolve we will experiment with adopting even more.

There is, however, no expectation or any real plan that C++ will be gone from Firefox. There's a huge amount of code, and there's a cost to rewriting it.

I only claimed “safety critical areas”... the other millions of lines and OS interop for UI, etc., will most likely not change for a long time.