As far as I am aware the time has come and as of Firefox 72 XUL has been stripped from firefox and so the method used to inject this scrollbar theme is no longer supported -- reference the following for future scroll themes:
Mozilla is currently working to phase out the APIs used to make this theme work. I will try to maintain each version until that time but eventually there will be no workaround. When that time comes there is a new, but more limited api for applying simple themes to scrollbars. In nightly I am currently using the following userContent.css
:root{
scrollbar-width: thin;
scrollbar-color: rgb(82, 82, 82) rgb(31, 31, 31);
}