Try copying an image from image search. On Chrome there’s newer UI where you can long-press an image and save it or copy the url. While on Firefox without addons it opens up a legacy UI that blocks long-presses. You either have to visit the site itself and fish out the image there, or press share, open the link yourself, which opens even older image page, where you can copy the url from “Full-size image” link. Google claims that Firefox lacks some abilities necessary to display Chrome’s UI, but there’s a simple addon called “google search fixer” that just mimics chrome’s user-agent and proves that this is not at all the case.