Mozilla:help!

Hello, I am trying to modify mozilla. Could you help me?
I really need to write this line in my start.html:

netscape.security.PrivilegeManager.enablePrivilege("UniversalBrowserRead");

But Mozilla show me an alert like: "Allow" "Deny". I would like to allow 
the cookie FOREVER. But I want to do it in mozilla source code, I don't 
want to do it by Edit->Preferences->...

Do you know how can I do it?

Thanks