webkit-scrollbar not working in firefox

Hello there,

I’m working on a theme for a client and they want to use a styles scroll bar for the fixed height elements of the site.

I’ve decided to use webkit-scrollbar for thisand it works fine on everything apart from firefox. does anybody now what this might be?

the guilty culprit can be seen here https://nouvellelondon.com/londonvc/blog/

Ant help would be much appreciated!