how to lock preferences in firefox 3.0

I try to lock preferences in Firefox 3.0 beta 5 on ubuntu 8.04 RC but it doesn't work.
I've tried to update the /etc/firefox-3.0/pref/firefox.js:
lockPref("network.proxy.type", 1);

but it doesn't work.
If I use "pref" instead of "lockPref" the setting is taken into account.
It worked with Firefox 2 (by modifying the /usr/lib/firefox/firefox.cfg file)