Thumbnails - why have they suddenly appeared?

About three weeks ago and out of the blue, all my Adobe files changed from the adobe icon to a thumbnail (which I do not want), and at the same time, the cursor turns into a black square instead of the arrow when navigating around certain sections of the page.

It is very difficult to control, particularly when trying to 'pinpoint' a specific tab, X or similar.

I did a search about the 'black square' and tried to follow it, thinking that perhaps it's something to do with the settings, but am at a loss as to how to change this.