I've noticed recently on my machine (Windows 7, 64 bit) running FireFox 19.0.2 I get strange holes over my browser and sites. This happens with in StackExchange Chat, and also when I'm on the Facebook home page.
It always appears on the right hand side, along the top like in the image below, or at the bottom corner just above my task bar and system clock. It might be flash issues although I really don't know and I can't find anything else online about it. I've got Adblock Plus installed, and nothing else.
The same setup on my work machine (Windows 7, FireFox 19) does not have this issue. Also, it doesn't do this all the time, on every page.