/usr/share/xfce4/doc/fr/index.html doesn't contain the encodage information, and so firefox detect bad the encodage. This page should have <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> But all the other page are in ISO-8859-1, the index should be in ISO-8859-1 too, no? And there is a translation mistake: Manuals -> Manuels
Fixed in the current repositories (both 4.2, trunk), thanks.