I hope to use Firefox on SLED 12.2 with old fashioned scrollbars. Firefox uses GTK 2.0. Therefore, I created the ~/.themes/Adwaita/gtk-2.0/gtkrc with the following contents:

include "/usr/share/themes/Adwaita/gtk-2.0/gtkrc"
gtk-primary-button-warps-slider = 0
style "scrollbars"
   GtkRange::slider-width = 16
   GtkScrollbar::has-forward-stepper = 1
   GtkScrollbar::has-backward-stepper = 1
gtk-primary-button-warps-slider = 0 solves one of the problems. Now left-clicking above or below the slider moves one page up or down. Without this configuration, left-clicking moves the page to the position of the click confusingly.

GtkRange::slider-width = 16 enlarges scrollbars and sliders for my fatty fingers.

GtkScrollbar::has-forward-stepper = 1 and GtkScrollbar::has-backward-stepper = 1 have no effect on Firefox 45 on SLED 12.2, unfortunately. But they have the desired effect on Firefox 45 and 52 on RHEL 7.3 running GNOME 3.14. There they bring back scrollbar arrows. How can I accomplish the same on SLED 12.2 running GNOME 3.20?