It sounds like that your connection is dropping out while attempting to download the file. Does this happen with every larage file you download, or just some of them?

Another thing you could try is to see if the download works in Safe Mode: http://support.mozilla.com/en-US/kb/Safe+Mode

If you continue to experience the same issue in Safe Mode, it could be an issue with your internet connection or Wifi.

You're using Firefox 16, which was released on October 9, 2012. Please update to the latest Firefox 27 version ASAP.

It is possible that anti-virus software is corrupting downloaded files or is interfering otherwise.

Try to disable the real-time (live) scanning of files in your anti-virus software temporarily to see if that makes downloading work.

See:

• http://kb.mozillazine.org/Unable_to_save_or_download_files
• browser.download.manager.scanWhenDone = false