Currently there is no “always on top” button for windows on XFCE Desktop Enviroment, there is a option to make the window always on top but only accessible through a window menu. Such button is on a wish list for future XFCE improvements… until then what we can do is create a keyboard shortcut for executing a command terminal that puts the current focused window always on top of all others.
Go to Keyboard Preferences and create a shortcut that runs the following command:
wmctrl -r :ACTIVE: -b toggle,aboveDONE !