Dark scrollbar for Firefox 57. Tested on Windows 10. Fixed element scrollbars and resize on page zoom. Requires https://gist.github.com/Sporif/db6b3440fba0b1bcf5477afacf93f875

dark-scrollbar.as.css

scrollbar, scrollbar *, scrollcorner {
	-moz-appearance: none !important;
	--scrollbar-width: 0.67vw;
    --scrollbar-height: var(--scrollbar-width);
}

scrollbar, scrollcorner  {
    background: #282828 !important;
}
scrollbar[orient="vertical"] {
    width: var(--scrollbar-width) !important;
    min-width: var(--scrollbar-width) !important;
}
scrollbar[orient="horizontal"] {
    height: var(--scrollbar-height) !important;
    min-height: var(--scrollbar-height) !important;
}

scrollbar thumb {
	background: #646464 !important;
}
scrollbar thumb:hover,
scrollbar thumb:active {
	background: #00ACED !important;
}

scrollbar scrollbarbutton {
    width: var(--scrollbar-width) !important;
    min-width: var(--scrollbar-width) !important;
}

scrollbar:hover scrollbarbutton {
    background-color: #444444 !important;
}

scrollbar scrollbarbutton:hover {
    background: #646464 !important;
}

scrollbar scrollbarbutton:active {
    background: #00ACED !important;
}

Is 0.67vw, the default scrollbar width of Firefox?

Is 0.67vw, the default scrollbar width of Firefox?

The default width of the Firefox scrollbar is 17px, like on most other browsers. The default scrollbar (unlike a custom one) is not affected by resizing tho. I just chose 0.67vw because it was roughly the same size as Sporif's version on a 1080p screen.
If your screen has a different resolution it will have a different size, so just change it to whatever size you need it.

Okay thanks. I can confirm that this works for Firefox 70.0.1 on Windows 10.

Works great, thanks @Xintaur!