I've been playing with Rust lately, and since I mostly work on the Add-on Manager these days, I thought I'd combine these into a toy rust version.
One of the core components is a class named AddonInstall which implements a state machine to download, verify, and install add-ons. The main purpose of this toy Rust project so far has been to model the design and see what it looks like.
So far mostly it's an exercise in how awesome Enum is compared to the JS equivalent (int constants), and how nice match is (versus switch statements).
It's possible to compile the Rust app to a native binary, or alternatively to asm.js/wasm, so one thing I'd like to try soon is loading a wasm version of this Rust app inside a Firefox JSM (which is the type of JS module used for internal Firefox code).
There's a webplatform crate on crates.io that enables which allows for easy DOM access, it'd be interesting to see if this works for Firefox chrome code too.