C and Rust? Firefox looks like a ton of C++ to me: https://github.com/mozilla/gecko-dev

Primarily C++, but Rust is there as well. However it's mostly libraries which were extracted from gecko itself, so it's dependencies, not main codebase.

I stand completely corrected, don’t know why I had that in my head.

The vast, vast majority of the Firefox codebase is C++.

You're thinking of Servo - https://servo.org/

Firefox now includes many rust components.