This is something I've only noticed a couple of times, but it's odd. Recently I got a new laptop and I'm using Firefox. I also just recently changed antivirus from McAfee (the machine had a trial) to AVG Antivirus.
Sometimes when I'm on E, I'll see black square pixels in seemingly random places throughout the screen, like someone has drawn a scattering of black dots on the screen. This has only happened 2 or 3 times and when I click on to another page it's gone. I haven't noticed this for any other sites.
Googling it, it looks like Mozilla has a bug in relation to Facebook that is described in a way where it might be similar, but I just thought I'd let you know that I'm having this problem in conjunction with here.