The missing scrollbar arrows tells me you likely have a GTK2 theme in use on your Linux distro instead of a GTK3 theme.

The official builds of Firefox since 46.0 have required GTK 3.4 (three.four) at minimum to run (though most people should have at least 3.12 by now) and as a result you needs GTK3 themes to theme Firefox. Firefox can technically be used with a GTK2 theme in use however Firefox will look unthemed and have some issues like missing scrollbar arrows.

Some Linux distros may have delayed this change to use GTK3 with their own packaged builds of Firefox until more recently.

GTK3 themes may be available to install in your package manager or you can get them from say https://www.gnome-look.org/browse/cat/135/ord/latest/

I have Mint 17.3 running on another computer, with Firefox 61.0.01, and it has the arrow buttons. I find it odd that it okay on an older version and not a newer one. Also, Chromium has the buttons on both versions of Mint. I guess I will go to Chromium. Thanks for your help.

You can try to create/edit a gtk.css file in your home directory.

• ~/.config/gtk-3.0/gtk.css

.scrollbar {
-GtkScrollbar-has-backward-stepper: 1;
-GtkScrollbar-has-forward-stepper: 1;
-GtkRange-slider-width: 15;
-GtkRange-stepper-size: 15;
}