Some installations of Firefox may block non standard HTTP ports.
To override these blocks, type this in the address bar about:config and search for network.security.ports.banned.override in the "Preference Name" column.
If network.security.ports.banned.override does not exist, right click anywhere in the window and select new->string and use network.security.ports.banned.override as the Preference Name then add the port number(s) you need to connect to as the value. You can also select a range of ports by using 1-1024 as such a value or as a CSV style values such as 1024, 8080, 1-1000, 80 et cetera.