Thank you very very much for the site ! I programmed something behaving
similar in FreeBASIC and one in Gambas. I will send the site to some friends,
but I will warn them it is a nasty surprise !
I opened a terminal window and gave the

xkill

command and then clicked on the moving target and it was gone. Then when I
started Firefox I chose "Start new session" instead of "Restore previous
session".