> Maybe an argument could be made that some security issues, for example, have been avoided thanks to Rust, but that's not really provable in any meaningful way.

Google has some pretty good data on this: https://security.googleblog.com/2022/12/memory-safe-language...

As for Firefox, the CSS engine was rewritten to be parallel, among other improvements (new wasm compiler, new parallel compositor) that would have been challenging without Rust: https://wiki.mozilla.org/Oxidation