I didn't understood if you absolutely want to use that mozilla download manager. It is more easier with wget, and its -c (continue) option
My i-net connection isn't very nice, too, here is how I use wget:
wget -t0 -T30 -c http://the.link/goes/here
-t0 <= unlimited tries
-T30 <= if there isn't a recieving in 30 seconds, retry
-c <= continue (resume) pervous attempt.
There are many more options, just see man wget. You can fall in love with wget