You don't explicitly say, but I assume, that the Firefox client of which you have posted an image is running on the same machine that is hosting Tvheadend. Confirmation would be useful.
Assuming this to be the case, have you tried:
http://127.0.0.1:9981
or
http://<ip address of Tvheadend box on local LAN>:9981
Actually, the easiest thing to do is issue the following command from a terminal window on the Tvheadend host:
sudo netstat -tulpen | grep tvh
Post the output to the above command. It should show what address(es)/port(s) tvheadend is listening on.
