Here is my report and solution (temparary?) after one week of searching and trying.
In (K)Ubuntu it is working without any problem in FF with Totem browserplugin 2.24.3 + GStreamer 0.10.21
To get it working in Kubuntu isn't easy, but it is possible.
In OpenSuse 11.1 Gnome with the same plugin and restricted formats
it isn't working.
To get it working in Pardus (and Suse) apply the folowing changes:
Add in FF the MediaPlayerConnectivity Add-ons.
Check in Package Manager the installation of "vlc".
Change in MediaPlayerConnectivity the player used for 'Windows Media' into /usr/bin/vlc and the video is shown in an extenal VLC screen.
This is not the perfect solution but at the end the video is shown in FF.