try this
irc://irc.mozilla.org/chatzilla
There you should be able to find developers of chatzilla and ask questions
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Here is some additional info on how to connect
http://www.hacksrus.com/~ginda/chatzilla/faq/#connect
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not postitive on this but....
I think you can launch Xchat
pick your nickname
select a network and select connect
Then you need to join a channel - go to server -> join channel
if you don't know your channel go to -> window -> channel list -> refresh the list
just look for the #icq channel