I think I can give you some more information. I have two monitors, one is a TV (Samsung UE32J5200AW) and one is a PC monitor (LG 29WK600-W). The TV is on HDMI, PC monitor on DisplayPort, but using the PC monitor on HDMI makes no difference.
When both monitor are activated in OS, there are no glitches or white spots.
When I switch off the second monitor and I set only the PC monitor to be used in Windows, the white spots and glitches appear in Firefox on the only active screen, the PC monitor, and on other applications too. When I set both monitor again to be activated in Windows, there is no problem at all, with or without hardware acceleration.
I think the driver fails to manage correctly dual monitor configurations. The problem will not go away, even if I disconnect the TV and try to use only the PC monitor.