The Tor Browser, a modified version of the Firefox browser with many privacy-enhancing features, will include more code written in the Rust programming language. Rust is a safer version of C++ that makes it more difficult for developers to accidentally introduce memory corruption errors in their code. The decision was to slowly start using Rust to replace the C++ code they used on top of Firefox codebase. Other plans include adding a non-trivial Rust API and adding automated tooling for Rust, such as linting and testing.
Source: https://www.bleepingcomputer.com/news/software/tor-browser-will-feature-more-rust-code/