In my comment I actually wasn't talking about parallel Rust providing a performance benefit over single-threaded C++, I was referring to how, by writing their codebase in two different languages, they're missing out on theoretical optimization potential (via inlining) and thereby leaving performance on the table wherever Rust code interfaces with C++ code. By enabling cross-language inlining via LLVM, it should ideally erase any theoretical optimization penalty incurred by writing Firefox in two languages.