Our .cfg file contains the following: //Firefox Configuration lockPref("network.proxy.autoconfig_url","url"); lockPref("network.proxy.type", 2); we find that when firefox is launched, the .cfg file is modified (network.proxy.type line is removed) to contain the following: // //Firefox Configuration lockPref("network.proxy.autoconfig_url","url"); lockPref("security.use_mozillapkix_verification", false); lockPref("extensions.install.requireBuiltInCerts", false); The end result is that network proxy settings in options>general>network proxy are greyed out. the Automatic Proxy Configuration URL is configured, but the option to use system proxy settings is selected. in about:config the network.proxy.autoconfig_url status is locked and has the correct value, in about:config the network.proxy.type status is still set to default, and is not locked. We want to specify the Automatic Proxy config url and then lock it down to use this rather than 'use system proxy settings' However, whatever we try, the netwrok.proxy.type line is removed from the config, as soon as firefox is launched. Thanks,