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?

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!