The current plan is to write a new browser called Servo in Rust, which is not intended to supplant Firefox. Servo is currently classified as a research project, not product development, with the goal of exploring parallelism and safety in browser engines. Small, isolated utilities written in Rust may end up in Firefox.
(I didn't downvote you, btw. I'm guessing someone thought you were being needlessly sensationalist.)
I think the plan is that FirefoxOS and Firefox for Android will transition over to Servo, presumably because of the major benefits of parallelism on mobile. Desktop, as you say, it's just limited modules.
(I didn't downvote you, btw. I'm guessing someone thought you were being needlessly sensationalist.)