iceweasel on debian docky problem

# Iceweasel 4

manu@leonard:/usr/share/applications$ xprop | grep WM_CLASS | awk '{print $4}'
"Iceweasel"
manu@leonard:/usr/share/applications$ grep StartupWMClass iceweasel.desktop
StartupWMClass=Firefox-bin

# Iceweasel 6
manu@leonard:/usr/share/applications$ xprop | grep WM_CLASS | awk '{print $4}'
"Firefox"
manu@leonard:/usr/share/applications$ grep StartupWMClass iceweasel.desktop
StartupWMClass=Firefox-bin