X-Chat for Windows
Pidgin - irc, MSN, ICQ, Google talk

Chatzilla

telnet irc.sioff.net 6667
PASS *
NICK tester
USER tester 8 * :testtester
JOIN #slo-tech
PRIVMSG #slo-tech Pozdravljeni.
QUIT

networks.txt from firefox\profiles\chatzila:

START <Array>
    START 0
        "name" "ircnetworkone"
        START <Array> servers
            START 0
                "hostname" "bouncer"
                "port" 57777
                "isSecure" true
            END
        END
    END
    START 1
        "name" "ircnetworktwo"
        START <Array> servers
            START 0
                "hostname" "bouncer"
                "port" 57777
                "isSecure" true
            END
        END
    END
END 

Global Settings | Lists | “Enter any commands to be run when starting chatzilla”

/attach ircs://ircnetworkone/?pass=passforone
/attach ircs://ircnetworktwo/?pass=passfortwo

After first connect use “Open this network at startup”. Restart chatzilla and edit/set proper irc USERNAME for ircnetworkone and ircnetworktwo.