Hmmm... It works for me on both CentOS 7 (Firefox 60.2.1) and Fedora 28 (Firefox 62.0.3).
As per the linked documentation - What happens if you SHIFT-CLICK on the scroll bar? On my systems I find that SHIFT-CLICK on the scroll bar produces the "warp speed" behavior, exactly as expected.
https://developer.gnome.org/gtk3/unstable/GtkSettings.html#GtkSettings--gtk-...
In Firefox 45.4, shift-click reverses the behavior, so it scrolls a page
at a time. It ignores the primary warp setting. I also checked in Terminal, and it respects that setting. With primary warp turned off, it moves a page at a time, regardless of whether I hold shift. With primary warp turned on, it warps regardless of whether I hold shift. I'll try updating Firefox.