js-files are ignored - one profile for all users

I have to prepare Mozilla for a migration on a lots of clients with an
software distribution system. Mozilla will be installed on 'clean' NT
mashines and on mashines with Netscape 4.75 on it with existing
Netscape profiles.
My goal is to set Default Mozilla Settings for clean mashines with no
existing profiles on it and in case of existing profiles ask the user
if he wants to convert the Netscape profiles to Mozilla.

My problems:
- Mozilla ignores my edited settings in the *.js files in the
defaults/pref directory! - Can someone tell me why?
- Is there a possibility to set up Mozilla so the default profile is
in the 'all users' directory?
- Is there a possibility to set up Mozilla so that ALL users have the
SAME profile?

Thanks for your answers
Frank