When working background change, web browser, or any other stuffs related to desktop environment the system get freeze... no more changes... then its necessary to shutdown the system by using power button only.
Sounds the problem is related to Xorg itself. You can try to start an Xorg session with 'twm' for example (or 'xterm') and run a webbrowser... but I'm sure this is not related to a specific Xfce component, therefore closing as INVALID.