I got the weirdest of errors today trying to open a web page on port 6000:

After reading for a while I found out that de Mozilla Foundation built in this protection for “Cross-Protocol” scripting attack with a form of Port Banning.

To overide this protection use one of the following steps:

• In the user’s profile directory the all.js, add the following line at the end of the file user_pref("network.security.ports.banned.override", "1-65535");
• In the defaults/pref/ sub-directory of the installation directory (multi-user systems) add the following line at the end of the file user_pref("network.security.ports.banned.override","1-65535");
• Open a new window, in the address type: about:config and add a new entry of the type string with this name network.security.ports.banned.override and value 1-65535.

If you want to set free only one port change the range for that port o list of ports.