Besides the fact that everyone has already pointed out about the thumbnails being gone. I have a different issue. Stuttering during video playback on any flash or html 5 video. Just to make absolutely sure it was this add on I first restarted firefox in safe mode, stuttering gone. Then one by one I disabled an add on until the stuttering and thumbnails returned. It was this one, please fix I really enjoy blocking certain youtube channels from my children.