By default Firefox has some ports blocked to prevent exploits to vulnerable services. This proves problematic if you need to access a site that uses one of these ports. For example, port 6000 is blocked because X11 uses it, but if you had a workstation running VNC’s HTTP server on port 6000 Firefox would block your access.

To unblock a port in Firefox goto about:config in the address bar.

Look for the following key: network.security.ports.banned.override

If the key isn’t present you need to create it. Right-click in the about:config window. You’ll want to click New and the String.

Give it the name of: network.security.ports.banned.override

Give it the value of the port you want to allow such as 6000. If you want to unblock more than one port use a comma to separate the ports.