changing print preferences using javascript

Greetings,

I would like to modify Mozilla's print header and footer in a javascript
function. I've written the following code:

function printMe()
{
    const secEverything = "UniversalXPConnect " +
                          "UniversalFileRead " +
                          "UniversalBrowserRead " +
                          "UniversalPreferencesRead " +
                          "UniversalPreferencesWrite " +
                          "UniversalBrowserWrite"

    netscape.security.PrivilegeManager.enablePrivilege(secEverything)
    var prefs =
Components.classes["@mozilla.org/preferences-service;1"].getService(Componen
ts.interfaces.nsIPrefBranch)


    // Change headerleft
    var headerLeft = "I am new header left"
    prefs.setCharPref("print.print_headerleft",headerleft)
    var newHeaderLeft =

    // Change homepage (for testing purposes only)
    var homepage = I would like to modify Mozilla's print header and footer
in a javascript function. I've written the following code:

function printMe()
{
    const secEverything = "UniversalXPConnect " +
                          "UniversalFileRead " +
                          "UniversalBrowserRead " +
                          "UniversalPreferencesRead " +
                          "UniversalPreferencesWrite " +
                          "UniversalBrowserWrite"

    netscape.security.PrivilegeManager.enablePrivilege(secEverything)
    var prefs =
Components.classes["@mozilla.org/preferences-service;1"].getService(Componen
ts.interfaces.nsIPrefBranch)


    // Change headerleft
    var headerleft = "&U"
    prefs.setCharPref("print.print_headerleft",headerleft)
    var newHeaderleft = prefs.getCharPref("print.print_headerleft")

    // Change homepage (for testing purposes only)
    var homepage = "http://www.yahoo.com"
    prefs.setCharPref("browser.startup.homepage", homepage)
    var newHomepage = prefs.getCharPref("browser.startup.homepage");

}


After running the above function, the the variable vewHomepage contains
http://www.yahoo.com and if I click on Edit | Preferences I can see that the
homepage has been changed. However, while newHeaderleft contains the value
"&U", going to File | Page Setup | Margins & Header/Footer shows that the
left header did not change.

Any help would be much obliged.

Steve