Set network.auth.use-sspi in Firefox with Group Policy

I have downloaded the Group Policy templates and copied them to the appropriate location.

In gpedit.msc I have set:

Computer Configuration > Administrative Templates > Mozilla > Firefox > Authentication > SPNEGO

to include the required domain names and can see this reflected in Firefox when I open about:config.

However I also need to set network.auth.use-sspi to false and cannot work out which Group Policy setting does this. Has anyone else been able to configure this?

Use the older "autoconfig" mechanism, which can be deployed through GPO "Preferences→Files" and can be used to enforce any possible about:config setting.

1. First stage: Deploy %ProgramFiles%\Mozilla Firefox\defaults\pref\autoconfig.js with these two lines only:

   pref("general.config.filename", "mozilla.cfg.js");
   pref("general.config.obscure_value", 0);
   

2. Second stage: Deploy a file to %ProgramFiles%\Mozilla Firefox\mozilla.cfg.js with all settings that you want to enforce (formatted the same as the prefs.js file in your Firefox profile folder):

   // This file *must* start with a comment line.
   lockPref("network.auth.use-sspi", false);
   

(It is possible to add a third stage and make Firefox download further settings via HTTPS, by defining the autoadmin.global_config_url setting in the 2nd stage config. This is not necessary though, as the 2nd stage file will already be refreshed through GPO.)

Starting with Firefox 81 and Firefox ESR 78.3 there is a way to set a lot of preferences using GPO or policies.json (including those with "network." prefix): Mozilla policy templates \ Preferences

• GPO / gpedit.msc: Computer Configuration > Administrative Templates > Mozilla > Firefox > Preferences

{
   "network.auth.use-sspi": {
       "Value": false,
       "Status": "locked"
   }
}

• policies.json:

{
  "policies": {
    "Preferences": {
        "network.auth.use-sspi": {
            "Value": false,
            "Status": "locked"
        }
    }
  }
}

If Preferences setting is used in both policies.json and GPO then only settings in GPO are used. Applied policies can be checked on the about:policies page.