This happens, because Tor Browser does not accept remote commands by default. See here and here for details. As mentioned in the tickets, you can work around this by starting the browser with the argument
--allow-remote. You will need to add this option when you start the first instance and when trying to open a link from an external application. To facilitate this, you can create a simple bash-script:
$FOLDER/start-tor-browser --allow-remote $@
Save this script in the Tor Browser folder (next to your
start-tor-browser file and grant execution rights. You can then use this script as your default browser and everything should work as expected. Keep in mind though, that the developers probably had a reason to make
--no-remote the default option. I think the main concern is, that apps on your system could open webpages via Tor and link your Tor identity with what they know about your system.