Confusing ... Rated 3 out of 5 stars
Clicking on the square icon to the upper right of that New Tab page hides the thumbnail pictures and your choice is remembered even after you restart Firefox. Problem is, it also hides your background image.
Note that the square icon remains displayed – if you click it again the thumbnail pictures will reappear on New Tab pages. As well as your background image.
The developer is racing after updating his add-on, but doesn't seem to be willing to consider having the background maintained when the thumbnails are hidden.
Well, now it shows when tabs hidden. You may check 13.15.63.