If a download fails, only some sites will allow the download to resume from where it left off. Go to the Mozilla Add-ons Web Page https://addons.mozilla.org/en-US/firefox/ (There’s a lot of good stuff here) and search for a download manager They work better than not having one. This download manager works separately from the browser. The browser tells it what to download, and off it goes. http://www.freedownloadmanager.org It is a powerful modern download accelerator and organizer for Windows and Mac.