Mozilla announced that the first Rust components will arrive in Firefox 48, scheduled for publication on August 2, 2016.
Development of Rust began seven years ago as a safer alternative to C++, to deliver a more secure browsing environment by reducing potential vulnerabilities like memory exploits. The first Rust component that will make it into Firefox 48 will be the Rust Media Parser:
The Rust Media Parser will be the first section of code to be applied to the main Firefox build. The parser was chosen because "every time a browser plays a seemingly innocuous video ... it's reading data delivered in a complex format and created by someone you don't know and don't trust".
As a result, watching video is one of the most vulnerable times in any browser user's experience, and moving it over to Rust will create a significantly safer environment.
Mozilla's own testing has shown that the Rust-based parser has identical results in terms of performance.