Firefox has always been built with DOM, JS, CSS technologies, as the linked article calls out. This change replaces a large proprietary extension, XBL, with a standard equivalent.
> XBL is a proprietary technology developed by Mozilla. We are solely responsible for the maintenance and improvement of XBL. Any time spent on doing this doesn’t contribute to the evolution of the Web so effectively we don’t spend anytime improving XBL except for plugging serious holes.
What they mean is they 'own' it, the good, the tedious and the bad parts.
Yes, that's what I meant by "you". But if you want to take a more active role in improving the web, you're free to interpret that "you" as actually referring to you.