Starting with Firefox 54, the Rust programming language will take a bigger role in the Firefox browser. The language is a safer version of programming languages like C and C++, the languages at the base of Firefox and most of today’s desktop software. Mozilla engineers have been slowly replacing more and more Firefox core components with Rust-based alternatives. The first Rust component was used in August 2016, when the language was used to rewrite the browser’s multimedia stack, the module that deals with rendering audio and video files. Mozilla hopes they’ll see a boost in performance in the future.
Source: https://www.bleepingcomputer.com/news/software/rust-programming-language-takes-more-central-role-in-firefox-development/