Using .xinitrc would be better. Just write
firefox
shutdown -h
in your .xinitrc. There will be no window manager, so you will have a borderless FF window.
Of course, you still need to be able to startx without entering a password. I don't know how that can be done.
If you want to play with .xinitrc and check how your script works, edit your .xinitrc to your taste (and temporarily remove the shutdown line), switch to console with Ctrl+Alt+F2 (or F3,...,F6), login and type
startx -- :1
This will open another X session which can be reached by Ctrl+Alt+F8 (the original session is at Ctrl+Alt+F7).
You may find this one interesting too:
http://www.linuxnet.com.tr/modules.p...hlight=xinitrc