Try Flashgot, the integrated download manager for Firefox

Even if you are running a fully up-to-date computer and have access to a high speed broadband, a download manager could still improve your download speeds and the reliability of your connection. Download managers will separate files into smaller pieces in order do download and manage them more efficiently than a single, larger file. Another benefit of using a download manager is that most of them enable you to resume broken connections and incomplete files.

Flashgot 1.1.9.8 is a download manager for Firefox that enables users to easily handle single or multiple file download. It ships with the usual download manger features such as the ability to resume downloads after your Internet connection is lost or broken and is incorporated with your browser so there's no need to run a separate download manager. Downloads can also apply to media files on the web, so you can download music and similar files to your desktop.

Flashgot 1.1.9.8 link.