Real Kiosk is a Firefox 2.0 - 3.0 extension that defaults to full screen, disables all menus, toolbars, key commands and right button menus. Alt+Home still takes you home. ...
... looks like what you need.
Debian Kiosk Howto
Section 4.7 covers what you are asking.
It is not unusual that someone wants to do something, but it turns out that they are thinking about it in an unhelpful way. As Pixalleny said, public terminal users don't expect privacy to be enforced by the computer - the ones I've seen either do not allow personal files at all, or are just plain permissive. As I said, it is simpler for the machine to overwrite /home/guest as part of the logout process. You'll want to do something like that when you shut-down at the end of the day, anyway.
I picked this up off a windows internet cafe that was running highly permissively - but they re-imaged the whole drive every night. This narrows the malware window... but I cannot help feeling that this approach is a strain on the equipment and doesn't scale very well.