> It's in your home folder, if you happen to have one. Only if he doesn't use a login manager, and uses startx instead. Even then, the system could just be using the system xinitrc in /etc/X11.
I must admit I am slightly puzzled as to what Midori should ever have to do with an .xinitrc file. What does Midori storing security certificates have to do with the user-specific X session startup file? It seems very fishy to me.
