I think the functionality provided by this add-on should come as standard in Firefox (or any browser, really). It makes managing, tracking, organising and finding downloaded files really convenient, and it's very customisable, so you can set it up however you like. My personal favourite features are audio alerts, popup "toast" messages, a toggleable bar showing the individual progress of each download (which can be placed on the top or the bottom of the browser window), and a history of downloaded files in case I need to re-download a file later. It's robust and useful, with pretty much no fluff or bloat, and I don't think any of the alternatives are quite as good, and I tried about half a dozen of them before I settled on Download Manager (S3).