Hi everyone, I'm from France and I come here because I have a very strange issue since a few days. With Firefox, when I scroll, I see these strange grey bars : When I go lower to quit the screen zone when to problem appears and I come back to it, everything is back to normal.. Sometime, it's even parts of the page (pieces of pictures and texts) which are retain on the screen.. I sroll down and I scroll up again, and nothing is left, everything is normal. I observe this phenomenon only with Firefox and Twitter. Everything seems to be okay with other browsers (Chrome, Opera, Safari..). I try to deactivate all the addons, to re-initialise Firefox.. same problem. I don't think it's an hardware issue, isn't it ? Configuration : OS X 10.10.4 iMac 27" late 2013 (bought december 2014) CPU : 3.5 GHz Intel Core i7 RAM : 32 Go 1600 MHz DDR3 GPU : Nvidia GeForce GTX 780M 4096 Mo Firefox 39.0 All the updates are done via App Store. Any ideas of the nature of the problem ? Thanks for your help. Regards.