Thanx Mark and PeteBest
Sniffer is active and detect my configuration
(User Agent) : mozilla/5.0 (x11; u; linux i686; fr; rv:1.8.1.14) gecko/20080418 ubuntu/7.10 (gutsy) firefox/2.0.0.14
It reconize my navigator as firefox or opera if I used it (sum of navigators=sum of visits but the sum of
OS isn't equal due to a 0 visit in linux bar not reporting in other
OS bar).