user.js, and setting print_footer_center <--- REQ 4 Help

I work as part of sales team for a small UK based electronic entertainment
company. Our stock ordering system is going through an overhaul as a result
of the increased demands being placed on the ordering system.

Part of this streamlining process involves the creation of printable
invoices from web pages and online forms. Privacy is an issue ,As these
invoices will eventually be dispatched to customers and obviously we wish to
protect certain information.

When printing web pages from within Netscape Navigator 7, the document URL
is printed (as is other information) in the header/footer of the printed
page.

It is possible to disable this feature on NN7 for PC by going:-

1. File-->Page Setup.
2. Select the "Margins & Header/Footer" tab
3. Changing the "Headers & Footers" roll-out menus accordingly

This feature is not present on the MAC version of NN7.

After a fair ammount of investigation I found a possible solution. It seemed
possible that by using the "user.js" file in the profiles directory to alter
the user preferences...

user_pref("print.print_footercenter","");
user_pref("print.print_footerleft","");
user_pref("print.print_footerright","");
user_pref("print.print_headerleft","");
user_pref("print.print_headercenter","");
user_pref("print.print_headerright","");

This does not seem to override the preferences set within the application
(tested on both MAC and PC) and I do not understand what is wrong. Is it
that these properties can be read but not set? I have tried using different
prefernces from the "prefs.js" file and all seems to work fine, apart from
those above.

How can I activate these peferences, override the default settings and stop
the headers/ footers from appearing on a printed page?

Any information would be greatfully appreciated.