Hi entrOpy - the last hint “navigate to /usr/bin/torbrowser” helped - thank you very much for that and your time!!
Those " network.protocol-handler.warn-external" were already set to true by default.
Now the only question left is how to get read of that warning:
“The following link will be opened in Tor Browser.
Be careful if Tor Browser is already running as your activities might get linked.” to speed things up a bit when hitting a hyperlink.