Lower right-hand corner of Firefox window frame; small white square; green "S" and blue "P." Click = diagonal red line superimposed over letters, upper right to lower left. What is this? What's difference between line and no line?

Nothing appears when I superimpose the cursor over the white square. Next to it is a dark-gray lock-shaped icon which, if I superimpose the cursor over it, tells me if the site I'm on is authenticated. These icons are in the lower right-hand corner of the Firefox window frame just to the left of the three diagonal lines which are used to expand or shrink the window.