you actually don't need to use the DCC fserver port 59, you should use the standard dcc usage in mIRC
if you set the router/firewall to use port 500X for dcc, remember to also tell mirc to use this port
view>options>connect>options>[advanced button]
set the dcc port or port range in here, and then mIRC will know to use the port/ports corresponding to what you set in XP
Hope this helps
ScoT