short followup:I just realized that it isn't possible to start TBB with the full path name...
$ /home/username/tor-browser_en-US/start-tor-browser.desktop ... /usr/bin/env: ./Browser/execdesktop: Datei oder Verzeichnis nicht gefunden
You can only run the browser if you change into the directory $ cd /home/username/tor-browser_en-US/ and from there execute the start file $ ./start-tor-browser.desktop
