Revive Firefox scrollbar arrows 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