While we would want this to work, for now it's partly broken. As explained on bug 1181561 comment 26: - settings.js loads SettingsRequestManager.jsm in parent - in content, add an observer against a setting - have chrome code living in content that uses SettingsService to set the same setting Expected: Content observer is triggered Actual: No observer is triggered on content side. A quick patch to avoid anyone loosing time on this issue will be to deny loading SettingsRequestManager in content process