Hi Elmar,
Many thx for your answer. I tried moving the libflashplayer to /root/.mozilla/plugins but it is still unsuccessfull. Any video from youTube does not start. It seems quite weird, as there is no error saying that the plugin is missing or something similar. When testing the "about:plugins" in the URL bar, this confirms that the flash lib is loaded.
I also tried moving the lib into /root/.mozilla/firefox/plugins, and in this case, Firefox ask me to load the appropriate flash plugin from adobe.
So this confirm that the lib is at the good place.
Any other ideas?
Fabian