The Firefox developers are very good professionals that came to the conclusion that all the tools they devised to write memory safe C and C++ aren't enough for writing a safe browser and a new start in the form of Rust and Servo is needed.
The Firefox roadmap already has plans for incrementally add code written in Rust.
That's good news as I still use Firefox. Btw, are there any good docs or blog articles on how it integrates into C or C++ code so well? I don't often see that in new languages. Leads me to wonder if there's lessons to learn there for another project.
A lot of it is simply that Rust has an equivalent amount of runtime to C and C++, so you don't have two runtimes fighting with each other. For example, when Rust had green threads, interop was much worse, due to needing to switch to a C stack, as well as the actual initialization of the runtime itself. Without it, it's as straightforward as https://news.ycombinator.com/item?id=11622257
The Firefox roadmap already has plans for incrementally add code written in Rust.