Multithreading isn't Rust's only performance benefit. Another is being able to write single-threaded code in a way that'd be too risky to do without the borrow

Multithreading isn't Rust's only performance benefit. Another is being able to write single-threaded code in a way that'd be too risky to do without the borrow checker making sure it's OK. (I.e. in C++ one would write something a bit slower but easier to reason about.) A cool upcoming single-threaded performance boost in Rust is std::simd, which provides portable (non-ISA-specific LLVM abstraction) SIMD that you can augment with ISA-specific operations where needed. Firefox doesn't use it yet, but Firefox already uses the older (to be obsoleted by std::simd) simd crate that provides the same thing a little differently.