Revive Firefox scrollbar on Ubuntu

If you are missing the scrollbar arrows/steppers in recent Firefox versions on Ubuntu/Mint the way to revive them is to edit or create a file in /home/user/.config/gtk-3.0/gtk.css and add:

scrollbar { -GtkScrollbar-has-backward-stepper: true; -GtkScrollbar-has-forward-stepper: true; }

Addition: Depending on your current settings you have to set in about:config

widget.non-native-theme.gtk.scrollbar.allow-buttons = true