Firefox JS engine is in C++ even though rust has been available for years now, they've said more than once that they have no plans on porting Spidermonkey to Rust.
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
It seems unlikely that there is any great conspiracy at play here to me... they found a good solution to a problem, they implemented it. Spidermonkey is a highly optimized carefully examined code base, it's not easy to replace wholesale or particularly low hanging fruit to be replaced with rust.
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.
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]
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.