ThumbsDown is a Firefox extension for bulk downloading images.
Say you have a web page with image thumbnails and you want to download the original full-size images with minimum hassle. With ThumbsDown you select the images you want to download simply by clicking their thumbnails and then just start the download. ThumbsDown will download the original images into a folder of your choice.
Things don't happen quite that automagically out-of-the-box. Before ThumbsDown is any use to you on any given website, you need create a script that resolves the full-size image URLs from the thumbnails. Scripts are created using a wizard or by hand. You can read more about scripts and creating them from User Guide.
Also, ThumbsDown is not a standalone extension. It uses an Firefox extension DownThemAll! to handle the actual downloading.