Restore Firefox scrollbar arrow buttons in Centos 7.4

Change the value of the following lines

GtkScrollbar::has-backward-stepper = 1
GtkScrollbar::has-forward-stepper = 1
GtkScrollbar::stepper-size = 13
GtkScrollbar::slider-width = 13

in /usr/share/themes/Adwaita/gtk-2.0/main.rc

and comment off

# Disable arrows for qt in scrollbars
# image {
# function = ARROW
# detail = “vscrollbar”
# }
# image {
# function = ARROW
# detail = “hscrollbar”
# }