Administrators define various lock settings for Firefox via a config file named mozilla.cfg (located in FireFox install directory). One has to edit this config file to unlock any of the locked settings. Then reload Firefox if it’s already running.
Many organizations restrict add-on installs by users using this method. Thus, when user attempts to install a add-on Firefox reports that “Software installation has been disabled by your system administrator.”
To unlock the add-on installation feature one has to modify
lockPref(“xpinstall.enabled”, false); to
Refer to http://www.pcc-services.com/kixtart/firefox-lockdown.html for detailed list of settings.