Firefox windows no longer showing

When I hover over Firefox on desktop, it used to show all open windows (with pics of page,) - all of a sudden (guess I did something) it now shows just vertical list.

How do I get the pics back?

Thanks,

Paul

 

I just clicked the individual lines and the name came up.
It used to open all windows at once, but I guess this is ok.

 

Except now, after closing a window, the pics are all back.:confused