Try this integrated download manager for Firefox

Even with a fully up-to-date computer and high speed broadband connection; a good download manager could enhance your downloading experience. Download managers will separate files into smaller pieces in order do download them more efficiently than a single, larger file. Another benefit of using a download manager is that they enable you to resume broken connections and incomplete files.

Flashgot 1.1.7.2 is a download manager addon for Firefox that enables you to effectively handle single or multiple downloads. It ships with the usual download manger features such as the ability to resume downloads after your Internet connection is broken. It's incorporated with your Firefox browser so there's no need to run a separate program.

Flashgot 1.1.7.2 link.