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How to disable Mozilla Firefox automatic connections at startup

Usage of policies.json

You can specifiy firefox policies in a file called policies.json, you may have to create it as it is noy present by default.

Path to policies.json depending on your OS (you may have to create directories of the paths):

Windows : C:\Program Files\Mozilla Firefox\distribution\policies.json
Linux : /usr/lib/firefox/distribution/policies.json
MacOS : /Applications/Firefox.app/Contents/Resources/distribution/policies.json

You can find all the policies templates, according to your firefox version, on the mozilla github page : https://github.com/mozilla/policy-templates/releases

Usage of autoconfig.js and firefox.cfg

Some preferences cannot be set through the policies.json file, such as disable normandy, because of firefox restrictions. But we can bypass it by creating a JS file called autoconfig.js and a config file called firefox.cfg.

autoconfig.js is used to load and execute firefox.cfg.

autoconfig.js

// Setup config file
pref("general.config.filename", "firefox.cfg");
pref("general.config.obscure_value", 0);

Path to autoconfig.js depending on your OS (you may have to create directories of the paths):

Windows : C:\Program Files\Mozilla Firefox\defaults\pref\autoconfig.js
Linux : /usr/lib/firefox/defaults/pref/autoconfig.js
MacOS : /Applications/Firefox.app/Contents/Resources/defaults/pref/autoconfig.js

The locked preferences are specified in the firefox.cfg file.
Path to firefox.cfg depending on your OS (you may have to create directories of the paths):

Windows : C:\Program Files\Mozilla Firefox\firefox.cfg
Linux : /usr/lib/firefox/firefox.cfg
MacOS : /Applications/Firefox.app/Contents/Resources/firefox.cfg

Usage of proxy.pac file

Despite all the efforts to disable automatic connections via policies.json and firefox.cfg files, there where still a few connections that seems to be not possible to disable.
To bypass this phenomena, we will create a file named proxy.pac that will block access to the remaining URLs by redirecting them to an unreachable proxy.

Save proxy.pac on your machine and keep in mind the path to your file.

Once done, you should add the following line to the Proxy policy inside your policies.json file :
"AutoConfigURL": "file:///path/to/your/proxy.pac"

Disable startup connections

policies.json

URL Parameter(s) to set
https://location.services.mozilla.com "browser.region.network.url": ""
https://contile.services.mozilla.com
https://tiles-cdn.prod.ads.prod.webservices.mozgcp.net		 "browser.topsites.contile.enabled": false
"browser.topsites.contile.endpoint": ""
https://spocs.getpocket.com "browser.newtabpage.activity-stream.discoverystream.enabled": false
https://push.services.mozilla.com "dom.push.connection.enabled": false
https://accounts.firefox.com "browser.startup.homepage_override.mstone": "ignore"
(also disable news page)
https://shavar.services.mozilla.com "browser.safebrowsing.provider.mozilla.gethashURL": ""
"browser.safebrowsing.provider.mozilla.updateURL": ""
https://tracking-protection.cdn.mozilla.net "browser.safebrowsing.downloads.remote.enabled": false
http://detectportal.firefox.com "network.captive-portal-service.enabled": false
"network.connectivity-service.enabled": false
https://incoming.telemetry.mozilla.org
https://www.mozilla.org		 Set "DisableTelemetry" policy to true

firefox.cfg

URL Parameter(s) to set
https://normandy.cdn.mozilla.net
https://classify-client.services.mozilla.com		 lockPref("app.normandy.enabled", false)
lockPref("app.normandy.api_url", "")

Example of policies.json file

"policies": {
    "DisableTelemetry": true,
    "Preferences": {
        "browser.region.network.url": "",
        "browser.topsites.contile.endpoint": "",
        "browser.newtabpage.activity-stream.discoverystream.enabled": false,
        "browser.startup.homepage_override.mstone": "ignore",
        "browser.safebrowsing.provider.mozilla.gethashURL": "",
        "browser.safebrowsing.provider.mozilla.updateURL": "",
        "browser.safebrowsing.downloads.remote.enabled": false,
        "dom.push.connection.enabled": false,
        "network.captive-portal-service.enabled": false,
        "network.connectivity-service.enabled": false
    },
    "Proxy": {
        "Mode": "autoConfig",
        "AutoConfigURL": "file:///path/to/your/proxy.pac"
    }
}

Example of firefox.cfg file

// Disable Normandy service
lockPref("app.normandy.enabled", false);
lockPref("app.normandy.api_url", "");

Example of proxy.pac file

// File PAC
//
function FindProxyForURL(url, host)
{
  // just define hostname and unreachable port
  var var_blackhole = "127.0.0.1:12345"; // could be any other UNUSED port than 12345

  //URL deny definition - Blackhole for blocked URL

  if ( host == "firefox.settings.services.mozilla.com"
    || host == "content-signature-2.cdn.mozilla.net"
    || host == "firefox-settings-attachments.cdn.mozilla.net"
  ) {
    return "PROXY " + var_blackhole;
  }

  return  "DIRECT; ";
}

Sources