You mean, in the application maintained by the organization which devised Rust and is its main proponent? Someone I am not very impressed.

Rust was created to improve the situation where web browsers are written in C++. (I hope we can agree this is bad.) It was not created to impress you.

The combined total of Rust, C, and C++ is only about half of the Firefox codebase. So it's more like 20% (of the code that it might theoretically replace).

Because the code written in HTML and Javascript isn't going to be rewritten in Rust another language, probably ever.