The idea is that Rust is to Firefox what C is to Unix. It is a general purpose language, but one that is specifically designed to address the kind of problem you have when writing a web browser.
It was a Mozilla side project, but with Servo, totally in line with its mission of making a good web browser. I'd even say it was a necessary long term plan. Chrome was the better, more modern browser, and Firefox needed fundamental improvements like these to compete. If anything, I think they should have spent more energy on Rust and Servo, not less.
