Rust is a complicated language and still has a very long journey ahead to prove its worth, I have yet to see it being used extensively in Mozilla to re-write Firefox which hasn't happened yet (may not happen as replacing C++ with Rust is a nightmare given most useful systems code is still either C or C++).

Hopefully zig [1], picks up. It's compiled code is smaller and better in performance compared to Rust and also provides a mechanism to write safe code with allocator choices. Also the overall zig language design fits in brain as the grammar is not complex unlike Rust which has a steep and complex learning curve. Rust developer spend a lot of time learning language feature and fighting with borrow checker syntax and still need to rely on unsafe C library to do anything useful. Zig made a conscious choice to make it work with C and realize it needs to work with C rather than replace it unlike Rust which is relying on C and still trying to proclaim as C replacement, when its not yet ready.

[1] https://ziglang.org/

Parts of Firefox are already written in Rust:

https://wiki.mozilla.org/Oxidation

they are continuing the work to replace the least performant/secure components in Rust when it's easily possible to switch them out