FF, log out, get to the log in page
httrack, add url -> capture url -> popup has values
On FF it's tools -> options -> advanced -> settings
write down original values/ change values, OK
enter username/password and click log in.
httrack popup exits and Address box has new url.
change ff settings back or you will get unreachable error.

find the log out button and right click/copy link
httrack options -> scan rules past link make it rejected