You definitely sound like you come from a Windows background. Many OSX users don't even bother with their firewall being on.

You can download, good! Can you upload? Gnutella is a p2p (peer to peer) "sharing" network. So people who download are expected to share back. If nobody shared, you'd have nothing to download. Get the circle? The more who share, the easier it is to download the same & new items ... makes downloading faster.

1. Being behind a firewall can make a difference. I know when I upgraded from dial up to adsl, my initial results were not as good as they should have been. After port forwarding my router & getting rid of the firewall issue, my results improved greatly. Also, uploading helps connect to more sources & thus improves searches. Siemens Speedstream 4100 does have an internal NAT firewall. The question is whether your device supports UPnP or supports it reliably. Oh ... looking at Port Forwarding for the Siemens 4100 it seems it does support UPnP. What you need to do is enable UPnP. If UPnP doesn't work well, then port forwarding is the way to go. Port forwarding & UPnP are safe. The port is only being used whilst the program is open. And you already have other ports open so you can browse the internet.

2. I have found the Junk filter doesn't seem to work as well as it did. For that reason I now choose to show junk results at bottom of search results. In that way I can check them & junk or unjunk various results incorrectly determined by the filter. Move the junk filter slider further to left.

3. Same as you were when using dial up, except with added security with the NAT router. Other points already answered. Configuring your firewall be it OSX or NAT firewall is not totally opening the port ... it is specifically only allowing LW access.