That's how it has always worked, perhaps your website changed the way its image links work. This addon is an image downloader, not a thumbnail resolver. It has no idea if an image is a thumbnail link or not, it can't factually decide such things. Please use a different addon for such uses, and please update your review according to the addon's performance compared to its listed functions rather than that it lacks a tangential feature you wish it had.

PS: for many thumbnail links you don't need an addon, just alt-click the link to download whatever it points at. This may often be a webpage instead of an image though, and firefox may still need you to enable alt-clicking in its settings first.