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?