I've been reading reports of this, & have experienced it myself with black rectangles in the address bar, tool bar area of Firefox 20.0 . I've also had several instances where Flash stops working, with a message box asking if I want to turn it off for that page/site.
It *may* be ATI graphics only, may be caused by ATI hardware, drivers, Flash, &/or Firefox. It's a recent problem with some folks blaming the latest ATI driver beta, but I get it without that latest version installed.
At any rate just wanted to mention in case anyone has similar problems, just turn off hardware video acceleration in Flash & in all reports I've read it makes the problem go away. Myself, I just ignore it for the most part figuring I'll see what happens in a few days on Update Tuesday when Adobe normally releases their Flash update.