The developers claim it increases speed up to 400%, and while that may be something of a stretch, there's no doubting it's very quick. The software also has an option to download every link or image on a page hence the name. It detects absolutely anything on a page that can be downloaded and downloads it. You can use filters to see you just the files you want, select via simple check boxes which ones to download, custom rename them, create subdirectories, and more.
One feature thats notable missing is some kind scheduler so you can download at night (or while away from your computer) and not interfere with surfing. Still, it's a superb addition to Firefox. If you often need finer control over multiple, large downloads, then DownThemAll! is definitely worth checking out. The latest release, version 1.1.4, improves multi-language support and brings full compatibility with Firefox 3.5.x.