if they are just regular programs see:
http://www.debian-administration.org/articles/50
if it is daemons/services, go to
/etc/init.d
you can remove the ones listed there by using the command
update-rc.d -f program_script_name remove
Alternatively, you have the option in GNOME from the following:
menu>preference>sessions>startup..add command
As for starting Firefox in Kiosk mode, with limited access....I would have to check on that one.