Put this in your ~/.config/openbox/lubuntu-rc.xml insidein between <applications> tags:
<application class="Firefox" name="Navigator"> <fullscreen>yes</fullscreen> </application> To get specific class or name for your applications, use this:
obxprop | grep '^_OB_APP' and then click with your cursor oninside the app window.
Useful website: http://openbox.org/wiki/Help:Configuration