Not sure if you have noticed but... you are agreeing to a comment that celebrates Mozilla focusing in, and only in, its core competency and adding that they used a new language they created :-)
Browsers are already highly specialized and complex VMs. If you have the ability to build a browser and work on all parts of it, you have the ability to create new languages.
So creating a new language is not at all that far away from their core competency. They are also working in a space where they have a strong need for a good systems language, so the motivation is there as well.
