Well, the perfect downloaded that you were looking for is JDownloader. It is open-source, can download from multiple links from a file into a single folder automatically and intercept Firefox downloads (with an extension). I’ve never used a premium file sharing host, but JDownloader does have options to log in to your accounts on many hosts, so that if you have a premium account on one of them, you can get the higher download speeds or bypass any bandwidth limits by the host, etc… I’m not sure about downloading YouTube videos though, I don’t think JDownloader has a built in way to download YouTube videos (correct me if I’m wrong).