Normally, the page up key moves up a screen and the page down key moves down a screen. I hit some key combination inadvertently and now the page up key loads the previous page while the page down key loads a page I have returned from, mimicking the back and forward buttons. I can still use the space bar to go down a screen and the Home, End, and arrow keys work normally. The page keys work normally outside of Firefox. What did I do and how do I undo it?