As opposed to Quantum? https://www.techrepublic.com/article/mozilla-and-qualcomm-un...
The technology that article mentions is Microsoft's Intel->ARM translation, which is surprisingly good, and entirely unrelated to Firefox.
The "optimized" build they're talking about is almost certainly natively-compiled ARM64, and the reason it's hard is because Rust is built on an older llvm version...which doesn't have Google's patches that improve support for Windows ARM64.