Infernal Quack Le site d'un vilain petit canard

Install GreaseMonkey addon on Firefox : https://addons.mozilla.org/fr/firefox/addon/748

Then open the link http://infernal-quack.net/files/greasemonkey/geoportail_extras.user.js in Firefox. It will recognise that it is a GreaseMonkey script and ask if you want to install it.