Did you download the Torrent? Because the only thing you will receive if you don't have a proper bittorent client stuff (I don't know anything about it, never used it), would be the file which contains all the info the software needs to actually download the file you wanted. In other words: don't use torrent if you don't have a bittorent client. And thus: download directly from the FTP.

Secondly your PC may have decided to go into stand by mode: in which case it would have closed all (Internet) connections, and thus disrupted the downloading process - it appears you can resume downloading with Firefox. In that case: NP.