Seems firefox has a security system to stop you browsing sites using non standard ports. To bypass:
In the address bar type about:config
Search for network.security.ports.banned.override
If it dosnt exist, right click anywhere in the window and select new->string and use network.security.ports.banned.override as the preference name and the port number you need to connect to as the value and click the "OK" button.
Enter the port number or range in the box. i.e 80,81,82 or 80-200
Click "OK" to save