New Behavior on Up and Down keys

I'm used to scrolling up and down a web page using up and down arrows. Until a few days ago, when I pressed the down arrow key, the page would move down a bit; i.e., 4-5 lines if text and about the same amount if viewing an image. The up arrow key worked the same way except in the opposite direction. Now, on FireFox 10.0.1 on Windows 7 Professional, the first time I press the down arrow on a new page, it scrolls all the way to the bottom and then, if I press the up key, it returns me all the way to the top. If I then click the cursor inside the text - say at mid screen, and then press the down key the cursor moves down one line at a time and no scrolling occurs until the cursor is on the very bottom of the window. If I then try to scroll back up using the arrow key, the cursor moves up one line at a time and no scrolling occurs until it's on the top line. This happens only on Windows 7 Pro, not on XP nor my Linux (Ubuntu) machine. They continue to behave the (good) old way. Anybody know how to fix this?