I haven't used themes for a long time, but remember that under older versions of Firefox, when viewing the hundreds of available themes, one only had to hover the cursor over the theme image in order to see how it would look. I am running Firefox Version 63.0.3, and it appears that this preview is no longer available. However, the Firefox Help Script says, "To see what a background theme will look like before installing it, simply hover over it." Has some part of my Firefox installation become corrupted, or is the script no longer incorrect?