I spend a lot of time searching the net for infos how to make configurations that can't be change by users and take effect on user who already have started FF and so already have their "own" prefs.js in their profile.

Here is the solution:

https://developer.mozilla.org/en/Automatic_Mozilla_Configurator/Locked_config_settings

http://kb.mozillazine.org/Locking_preferences

For me it doesn't work with encoding, so I used

pref("general.config.obscure_value", 0);

pref("general.config.filename", "mozilla.cfg");

Like it is written at the bottom of the first link.