When watching a YouTube video, if you place the cursor on the loading bar, you can have a preview of the frame of the video at the second where the cursor is placed (see screenshot below). However, this preview is not always available, so how do you enable or disable this feature?
Video thumbnails cannot be disabled by the video uploader. Some videos indeed have no thumbnail but that's a glitch. Note that they can upload their own thumbnails if they want to.
For the viewers, YouTube Nativity has no option to disable video thumbnails, and the two main Google Chrome extensions (viz. YouTube Options and Magic Actions for YouTube) to customize YouTube has no option for it either.
I have noticed that the Loss of Preview is gone Due to "Shockwave Flash" , when I disable it on Mozilla Firefox , my Preview feature is back again just like before I added "Shockwave Flash" . So give that a go and see if it works !