I have a 2013 27" iMac and when I watch youtube videos, using Firefox, the control areas (top and bottom) of the youtube video flicker when I watch. This primarily happens at the top and bottom of the video where the controls and video header are located and not the main video. If I let the controls fade away then there is no problem, but as soon as I move the mouse the flickering is there again. This does not happen on my other Macs or on other video sites, only youtube. It started on Sierra and continues on High Sierra. I have done a clean install and I have scanned with Malwarebytes, which found nothing, but it still continues. This is only on Firefox, Safari works fine, and I do not have Flash installed. Any thoughts?