I think that you raise very valid questions. But first I'll start with some questions of my own. When and how is gnutella (Limewire) indicating that you are behind a firewall?

I do know that when you connect, it might tell you that you are a shielded leaf node. Which simply means that there's another computer aggregating your connection to the network. This is normal.
I'll assume that you are not talking about this message.

Regarding your router LinksysBEFSR41 I believe and I could be wrong that by default this device has DHCP enabled which means that your linux machine as a private net ip address (192.168.x.x or 10.x.x.x) You can verify this by opening a terminal on your linux machine and typing:

ifconfig

and look at your inet addr:

Is the ip of your machine a on a private net?
if yes (good) we're on the right track Because you are using NAT.

Regarding your default Linux installation. I think and again, I could be wrong, but by default Linux will install with your firewall set to medium. I think the choices are (no/medium/high/custom). If I'm correct and the firewall on your linux machine is medium this should not affect your filesharing abilities as the medium setting blocks inbound ports below 1023.

You can verify your firewall settings by using your gui interface /system settings/security levels

The changes that you made in Limewire should do it.

Regarding what happens when your isp changes your ip address. Unless something has changed in LimeWire since I last used it. I don't think the network supports: ip-id-config (term I made up to describe the process of determing the hosts ip and then configing the client with its public ip
address.) The logic being:

/* the client starts up and if the local ip is private-net then send a special message to a configuration server. */

{
client-x-ip-nat --> nat-router-public-ip --> ip-id-config-server
}

/* the server receives the configuration request from the client with public-ip-x i.e. NAT router. and then sends a response to the client indicating the client should force this ip in its configuration or automatically sets-it.- The nat router will forward this request back to the client because it knows that this returning information is in response to a client solicitation */

{
set-client-x-ip-to-public-ip --> nat-router-public-ip-->client-x-config
}

This would be an ideal situation -but is not the case.

So, you'll have to change it in Limewire to match your public ip.

I think that once you apply the changes they become active without you having to reset. However I think it might be a good idea to restart the application or at least disconnect/reconnect to the network. The reason I would make this recommendation is this. Again. unless something has changed (and I don't think a lot has.) Your machine sends to the network a list of the files you are sharing along with your ip address information.

I hope that this information helps you. If you need further assistance please stop by my website and email me directly for more information.

I should note that the worse that can happen is,
you'll be able to search and download stuff and your machine will return queryhits but no one will be able to download from you.

Hope this helps.

__________________
Lee Evans, President
LeeWare Development
http://www.leeware.com