Note that even on caniuse, this is described as a "non-standard method of styling scrollbars". Browsers have their own proprietary methods of styling them, but there is no standard. Therefore, I can't really blame Mozilla for not implementing a specification that doesn't exist.
Now, you may ask: "Why doesn't it exist?", to which I have no good answer. It's tricky to specify something that will work well across all possible scrolling mechanisms on all devices and OSes, which is similar to the reasons why we don't have a standard for styling form controls yet. It has been discussed briefly in the CSS WG though, and we do recognize that authors have a need for it, so I'm a little hopeful that we will see a spec for it in the near future. Fingers crossed!