This problem is solved.
First I used the command "nice -n 0 firefox ........" to force the launcher in
an other file.
Next I removed all the %2525...3B.desktop files.
And the gnome-panel was updated.
Result: the problem is gone.
** Changed in: gnome-panel (Ubuntu)
Status: New => Fix Released
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Multiple firefox launchers on panels get mixed up
https://bugs.launchpad.net/bugs/155472
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Desktop Bugs, which is a bug contact for gnome-panel in ubuntu.
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