Screenshot please : https://support.mozilla.org/en-US/kb/firefox-screenshots and be more descriptive about your issue regarding the screenshot.

I have realized this issue is unique to scrolling on a search results page generated by the Duck Duck go search engine (not google) so must be something in their delivery code that creates the shaded box behind each search result on the page as you scroll (impossible to screenshot). It interferes with my reading speed (lags behind). Solution is to switch to google but I thought it was a firefox cursor option missing. Will ask Duck Duck Go for a fix regardless.

There is a CSS rule to highlight the result:

• https://duckduckgo.com/r1519.css

.result.highlight {
 background-color:#f7f7f7
}

You can add code to the userContent.css file.

• http://kb.mozillazine.org/userContent.css

@-moz-document domain(duckduckgo.com) {
 .result.highlight {
   background-color: unset!important;
 }
}