From the rust Oxidization page (hip name):

- "Crossing the C++/Rust boundary can be difficult."

- "For components that are relatively standalone, with small and simple APIs."

- "This minimizes the C++/Rust boundary layer issues."

Not only does Rust not have a good JavaScript integration story, it doesn't have a good C++ integration story.

Most of Firefox is still (8 years later) not written in Rust.

Chrome with C++ has a waaaaay better safety record than Firefox. Guess what? The C++ in Chrome tries to ensure memory safety for JS.

The C++ Chrome tooling does a better job of memory safety as a whole system than Firefox.