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.

Thanks for the simple and honest summary. For fairness' sake, just know that firefox specifically does not even implement the download renaming API. Firefox addons could not add a missing extension even if the developer really wanted to.

Edit/reply: not in the way you think. Those addons determine their own filename from a bunch of guesswork. I consider that an anti-feature and want to leave the initial naming to the browser.