I would recommend the
wget command line utility which is absolutely awesome!
wget is a GNU free software package that is used for retrieving files over the web. If you have got it installed, then all you have to do is to get the download link and use wget to download it.
In order to get the download link, right click on whichever download you want in the firefox download manager. There will be an option ‘copy download link’. Click on it.
Then open a terminal and go to whichever folder you want the file downloaded to. Assuming it is in the Desktop, type the following command at the prompt:
$ wget <paste your download link here>
Now even if it gets stuck in the middle, you can resume from wherever it was interrupted by giving the option
wget. That is, the command will be like
$ wget -c <paste your download link here>
And there it continues beautifully.