Add sensible-browser to the browser list
Users on Debian-based systems who are running without a GUI might still have a text based web browser such as w3m or lynx. Debian gives us the sensible-browser command to launch one of these (either by environment variable or with the alternatives system). xdg-open still comes first, since GUI users will be less inclined to configure the sensible utils. This will also allow running the GUI web browser in a non-xdg system that doesn't specifically have firefox or chromium.