I have two behaviors.
when I go into URL capture mode, turn on proxy using the address/port
settings, Firefox times out when I submit login/password.

Reset HTTRACKER to default, reboot, clear firefox cache & cookies to try
again.
This time HTTRACKER capture URL opens, pauses, then closes. Repeat clicks
elicits the same behavior. 
It looks like the proxy server uses the same address while cycling a short
list of ports. Is this correct? Once it used 0, which firefox did not accept.
Are there any diagnostics for the proxy server? Conflict detection, etc.