Wondering what’s inside a YouTube video? The YouTube Thumbs add-on will cycle through different frames of a video as you hover your mouse over the default video thumbnail on the YouTube website.
YouTube allows video owners to upload custom thumbnail images and sometimes these thumbnails do not accurately represent the video. However, with the YouTube Thumbs extension installed, you are less likely to click on such “fake” videos.
Internally, it uses the three image frames that YouTube generates for every uploaded video and puts them in a loop as long as your mouse is over the thumbnail.