Se você tiver acesso root, sempre poderá editar o script startxfce4. Segundo a documentação:
startxfce4
The startxfce4 is a convenient script to start an Xfce 4 session from the console. It will give you a session with a taskbar and a panel and with the desktop manager and window manager running.
All programs, or symbolic links to programs, in ~/Desktop/Autostart/ will be run by startxfce4 on startup.
To customize the behaviour of startxfce4, copy the file ${sysconfdir}/xfce4/xinitrc to your personal ~/.config/xfce4/ directory and edit that file. If you install from source, ${sysconfdir} defaults to /usr/local/etc; for binary packages it is often set to /etc.
With the inclusion of a session manager in Xfce 4.2, the preferred way to change startup behaviour is by using the "Save session" option in the logout dialog.
Ou você pode usar o programa xfce4-autostart-editor
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Para a parte de logout, consulte a página link para o script de logout pessoal.