It must have something to do with your desktop settings because it does not
work that way for me and I am running the same Kubuntu and FF. Double click
opens a full screen window with no icons. Esc gets me back. I don't think
that it has to do with FF, but may have something to do with KDE and the way
you have it set up.

Do you have another desktop environment installed, such as GNOME? If so,
switch to that and try the same thing. If it happens then FF may be to blame
and if it doesn't then it is likely limited to KDE.

Do you have anything unusual installed that may give you an active desktop,
such as a TV card?

no i have only kde environment.
I will install another environment to check the problem.
but since i prefer kde, this won't resolve my problem.
there is no "unusual" program installed.
I succeeded to frame the bug:
the problem is that it works perfectly since i switch into fully screen
mode by clicking on the dedicated icon in the player.
but once i double click on the video the video goes to the background
and even if i try to use the icon another time the bug remains.

here is a link to check if it is a general bug or a misconfiguration of
my options:
mangas-tv.com/koukaku-no-regios-episode-017.html

What happens if you try the same thing in another browser? Rekonq comes with
Kubuntu. Try it in that.

Any chance VLC is involved? It's the only media player I know of that will play
a video as your wallpaper.

I think the default firefox player under kubuntu is totem.

Totem is GNOME. The default Kubuntu player is Amarok for Audio and Dragon
Player for video.