Want to see the thumbnail of a Youtube video in its highest resolution or original resolution? Quite simple!
In Firefox on the Youtube video page type "ctrl" + "u" (at the same time).
In the new window that pops up type "ctrl" + "f" (at the same time).
Now type "og:image". The searched text will light up green.
Right next to that highlighted text you will find the link to the original image in its highest resolution.