Mozilla is working on replacing the wasm compiler, and eventually the compiler for "ion MIR" (an intermediate langauge that js is compiled to), with cranelift, a rust implementation of a compiler backend. See [1] for pretty pictures of this.
It's been possible to use this backend to some extent since 2018 [2]. The best place I was able to find in the last 5 minutes to describe the current state of the integration is the doc comment at the top of this file [3].
In other words it looks like spidermonkey is not not being ported, it's just not being entirely ported yet.
[1] https://github.com/bytecodealliance/wasmtime/blob/master/cra...
[2] https://old.reddit.com/r/rust/comments/9mvnrk/in_firefox_nig...
[3] https://github.com/mozilla/gecko-dev/blob/master/js/src/wasm...
Edit: Another component of spidermonkey that is being rewritten in rust is the JS parser, that project is called smooshmonkey. You can read about it in the spidermonkey news letters [4] [5] [6], and there's an nice comment describing it at the top of this reddit thread [7]
[4] https://mozilla-spidermonkey.github.io/blog/2020/01/10/newsl...
[5] https://mozilla-spidermonkey.github.io/blog/2020/03/12/newsl...
[6] https://mozilla-spidermonkey.github.io/blog/2020/05/11/newsl...
[7] https://old.reddit.com/r/rust/comments/h0ddpi/smooshmonkey_n...
The most recent information I've found is from a Spidermonkey Dev here on HN who said they might even port the Regex engine to Rust
That didn't happen either
https://news.ycombinator.com/item?id=18988024
Something tells me that there's more than meets the eye.