How to use the Xfce4 Terminal drop-down feature
To use Xfce4 Terminal as a drop-down window, make sure you're running version 0.6.1 or newer. The terminal doesn't have a GUI option to run as a drop-down (it can only be used as a drop-down terminal by using the "--drop-down" command line parameter), so you must set a keyboard shortcut in your desktop environment to call "xfce4-terminal --drop-down":
in Xfce: open Settings Manager > Keyboard > Application shortcuts, click "Add" and add the "xfce4-terminal --drop-down" command, then a dialog will pop-up - now press the keyboard shortcut you want to use to open the drop-down terminal (I've used F12).