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.
That is not a built-in feature of Firefox, but will have been added by the SearchPreview add-on that you have installed. For details on what the add-on does see https://addons.mozilla.org/firefox/addon/searchpreview
Thanks for your reply. I have no idea how SearchPreview got installed and will uninstall it as soon as I can figure out how.