Rust is still very new. So far the only Rust code added to Firefox was a drop-in replacement for some other code. They haven't written any new features in Rust yet, and most FF devs wouldn't be fluent in it yet.
It seems to me that Rust would be perfect for a tiny, mostly irrelevant, self-contained feature like this. :-)
If it works, you don't need most FF devs to have fluency because the code base simply doesn't need fixing. The API surface is tiny. And the impact of it being broken is pretty low.
