I am testing using the Enterprise Deployment as per here https://developer.mozilla.org/en-US/Firefox/Enterprise_deployment I have created an autoconfig.js and placed it under C:\Program Files\Mozilla Firefox\defaults\pref. Here is the content: // pref("general.config.filename", "mozilla.cfg"); pref("general.config.obscure_value", 0); I have created a mozilla.cfg and placed it under C:\Program Files\Mozilla Firefox. Here is the content: // defaultPref("browser.startup.homepage","http://www.libraries.sa.gov.au/page.aspx?u=194"); defaultPref("browser.download.dir","d:\"); lockPref("browser.startup.homepage",0); lockPref("browser.download.dir",0); lockPref("browser.download.useDownloadDir",0); Here is the issue. With the above mozilla.cfg, Firefox doesn't start at all. Click to start and it just doesn't. If i comment out the two defaultPref lines it starts with the error as in attached image, displaying it three times (I assume once for each lockPref line). It starts successfully with the settings locked as per the lockPref settings. Can anyone see where the issue may be? thanks jcrow EDIT: Sorry, the question editor shows line breaks that don't appear in the posted question.