Changes
→Locking user preferences / Firefox settings for all users (like proxy etc.)
Then, create a file called mozilla.txt (yes, mozilla.txt, not
mozilla.cfg), and put your custom settings there(NOTE: this file has to begin with //, as the first line of this config file seem to be ignored):
<pre>
//
lockPref("network.proxy.backup.ftp", "your.proxy");
lockPref("network.proxy.backup.ftp_port", 3128);