I forgot to say: Opensuse Leap 42.2. Plasma 5.8.
It seems I have found the solution.
firefox and thunderbird are GTK applications, so its icons and configuration are dfined at system settings->Application style->Gnome application style (GTK), so they are gnome related.
So i force reinstallation with yast of all icons packages and most of gnome packages and now it seems to work fine.
regards