> When adding a file to share on the internet the link has the dedicated ip of my desktop for my router 192.168.1.1 and not the external ip address
menu > ip address > find external address
> when trying to test connection HFS seems to connect to various ports (51480, 80,81,...) most of them I have not forwarded.
yes, if your 51480 fails, it tries anyway some standard ports
> When trying to connect trough firefox using my external ip followed by :51480 there is nothing to connect.
YOU can't use your external address yourself. Only others can, those outside your LAN.