I had that problem too. I ran PCpitstop full system scan and it showed I needed a driver update for my video card. I accidentally downloaded the wrong one and when I opened up FF I had the exact same view as what you see. I took that driver out and got the correct update and things went back to normal.
Perhaps your system display got changed and you could fix it by right clicking on the desktop to get in to display properties and adjust your view...just a thought.