create a restricted user

[Zacharie Elcor]

I want to create a restricted user without password that can only use
a web browser.
I added a user named "visitor" and created in his home dir a file
.xsession that contains:

firefox

so that when he logs in, firefox is launched, and when he closes
firefox, he is logged out.
This works fine but he is still able to ctrl+alt+F(1-6) and log in to
browse the file system.

To prevent that, I tried to set /bin/false as the default shell for
that user in /etc/passwd but this also prevented him to log in
graphically.

Is there a way to be sure that "visitor" will only be able to browse
the web and not the file system ? any security issues ?

Thanks for help