Mozilla made a language in order to build their new browser engine, servo [1].
That language is Rust [2].
The reasoning was that we have been relying for too long on systems languages where memory safety [3] related bugs have caused a lot of exploits [C, C++].
In big codebases, like the browsers tend to be, bugs have many places to hide, and such bugs are very hard to spot. You can't rely on contributors being extra careful continuously, because everyone makes mistakes. So by using a programming language with a garbage collector or memory safety guarantees, like Rust, and enforcing extreme modularity, critical bugs are much easier to be spotted.
In big codebases, like the browsers tend to be, bugs have many places to hide, and such bugs are very hard to spot. You can't rely on contributors being extra careful continuously, because everyone makes mistakes. So by using a programming language with a garbage collector or memory safety guarantees, like Rust, and enforcing extreme modularity, critical bugs are much easier to be spotted.
[1] https://github.com/servo/servo [2] https://www.rust-lang.org/ [3] https://en.wikipedia.org/wiki/Memory_safety