There are what, 5+ rust javascript engines that claim to be production-ready? Bolting one of those on in place of spider monkey seems like a reasonable future direction
They're all more than 10x slower than SpiderMonkey.
They may be production-ready in some sense but they're not ready to be put in Firefox, and/or they are v8 bindings.
What do you mean by "production ready" here exactly? In a web browser context, the JS engine is expected to have a high performance optimising JIT compiler. Do the existing Rust JS engines have that?