I'm behind a router and nobody can connect to my server or transfer files / directory listings
You have to configure your router to forward the ftp port (default: 21) to the server machine. Further you have to forward a port range for passive mode transfers. You can specify this port range on the "passive mode settings" page in the settings dialog in the server interface. In most cases, a range like 5000-5100 is sufficient. Most likely, FileZilla can't determine the external IP address if behind a router. In this case you have to enter the IP address (or your host name) on the passive mode page in the settings dialog. In case you've a dynamic IP (most dial-up users), you can also choose to retrieve the ip address from a website.