Firefox’s download manager doesn’t have a built-in mechanism for resuming failed downloads. My Internet connection was cut off just long enough this morning for a Firefox download of an ISO image to fail. To resume the download, I used the wget command, which is built into most Linux distributions, and installable if you’re running OS X or Windows. If you’re running Windows you can download wget from GnuWin32. If you’re running OS X, you can install wget with DarwinPorts.
To resume the failed Firefox download, open up a terminal, change to the directory that the is located in, and issue the wget command with the -c option. The -c option tells wget to continue the failed Firefox download. For example:
wget -c http://download.mozilla.org/failed-download