How can I prevent the scroll bar from disappearing and keep it permanently on?

I am using the latest version of Firefox, and the scroll bar only appears when I place my mouse over it. Then I have to use arrow keys to scroll up and down the page, and it disappears very quickly. Once the scroll bar disappears not even the arrow keys work. Is there a way to turn the scroll bar on so that it is always on and does not disappear? I need to be able to scroll with both the mouse and the arrow keys any time I want to without moving the mouse over to the scroll bar. The disappearing scroll bar is incredibly annoying, and I cannot find a category for it anywhere in the settings menu.