Just wanted to document this here so it may help someone in the future.
A JetDirect J3113A (600N) with older 7.something firmware would not
upgrade to the latest version with HP Download Manager. It would start,
but then just sit there forever with the progress bar about 10% done.
Web JetAdmin took twenty minutes to install, then failed to start.
The 600N refused FTP connection attempts, so that method was out.
ftp-config:1 was not a recognized command. I have no idea how FTP was
disabled on it.
TFTP connections were recognized but always timed out.
HP Download Manager apparently uses TFTP to send the firmware. But, as
Bob Roell says in the link above:
"2. Make sure the network port matches the JD's speed/duplex setting.
TFTP does not handle excessive packet loss very well."
Aha! Old Ethernet stuff, autonegotiation, maybe a duplex mismatch?
Connecting the upgrading computer and the 600N to a newer switch solved
the problem. If that didn't work, I would have used a crossover cable
to connect the computer and printer directly.
JetDirect 600N firmware update 
Linear Mode
