Mozilla's Position on CSS Scope
github.com
github.com
You got Chrome and Webkit/Safari on board.
And then someone who just doesn't "get it" wasting three calendar years.
I make snarky comments about FireFox's refusal to-date to allow for "full" scrollbar style customization, but I'm serious. That coupled with inability to embed means FireFox could never be used in a system such as electron, and nobody developing "apps" but in the browser is happy with the inability to deal with those scrollbars.
It's like Mozilla/FireFox have completely lost the pulse of the web, or they just don't care about market share and would rather march to their own drum.