From what I know, your model of Linksys isn't too reliable in UPnP mode. You could try port forwarding - that will hopefully get rid of the firewall & might increase your download speed as well

The instructions are here: Port Forwarding for the Linksys WRT54G
Those instructions are for LimeWire but don't worry - setup's exactly the same for FW.
You'll need to set up a static IP address first - there's a link for that within those port forwarding instructions. FW's listening port, port in the 'manual port forward' box & port forwarded within the router's setup all need to be the same number

Rather than running without a firewall, it's better to use a software firewall & make sure that it's allowing FW. Check the link here, the instructions for LW are the same for FW: User Cant Connect - LimeWire

Also make sure that there's no Windstream firewall causing problems: http://www.gnutellaforums.com/218807-post7.html

You could also have a look here, for some tips to help performance (mainly the first few posts by murasame). Things are exactly the same for FW: A few things to consider while configuring Limewire for optimal performance...