I don't know if there's any one location for all the Rust code; it may not make logical sense to put the MP4 parser written in Rust right next to the CSS engine, for example. I believe that https://github.com/mozilla/gecko-dev is a mirror of the repo in question, and so we can use Github's search function to find every file that it detects as being a Rust file: https://github.com/mozilla/gecko-dev/search?l=rust (which finds 3,274 Rust files, 6,701 C files, and 22,722 C++ files).