It's basically a planned replacement for C++ as the language used to program the Mozilla apps themselves (and native add-ons). It wouldn't make sense as a scripting language that runs on Mozilla.
It may be that Rust is being designed as if that were a goal, but let's not start any rumors here.
Of course, if it turns out to work amazingly well and they can port all their old code (or interface with it) with relative ease, there's no reason not to jump ship from the hellfires of C++ compilation. They've had major issues with that lately, after all.