I find that feature quite buggy on my system. Firefox has redraw problems when it shows tooltip things or I use the middle click autoscroll (In Compiz).
I find it better to set up a keyboard shortcut in System -> Preferences -> Keyboard Shortcuts and under Window Management > Toggle Fullscreen. I used Crtl+F11.
This has the bonus of working in every program (KDE apps too) by just getting rid of the window borders and expanding it fullscreen.
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