improve Firefox default settings

- disable telemetry and studies, remove notice tab about telemetry
- force off DNS-over-HTTPS and always use the local dns resolver.
  This allows for example to access websites in the local part of a split dns setup
- set the default search engine to DuckDuckGo
- change the homepage to the internal firefox homepage (shows a searchbar and buttons).
  This doesn't load an external page (DuckDuckGo previously) without user consent/action,
  but still provides a convenient way to search via DuckDuckGo.

airootfs/opt/firefox-esr/distribution/policies.json

@@ -1,5 +1,10 @@
 {
   "policies": {
-    "DisableAppUpdate": true
+    "DisableAppUpdate": true,
+    "DisableFirefoxStudies": true,
+    "DisableTelemetry": true,
+    "SearchEngines": {
+      "Default": "DuckDuckGo"
+    }
   }
 }

airootfs/opt/firefox-esr/sysrescue.js

@@ -10,4 +10,13 @@ defaultPref("browser.shell.checkDefaultBrowser", false);
 pref("browser.EULA.override", true);
 // SystemRescue settings
 pref("browser.startup.homepage_override.mstone", "ignore");
-pref("browser.startup.homepage", "https://duckduckgo.com/");
+pref("browser.startup.homepage", "about:home");
+// disable Firefox telemetry and surveys, don't annoy the user with it
+pref("app.shield.optoutstudies.enabled", false);
+pref("datareporting.healthreport.uploadEnabled", false);
+pref("datareporting.policy.dataSubmissionEnabled", false);
+pref("datareporting.policy.dataSubmissionPolicyBypassNotification", true);
+// don't ever use DNS-over-HTTPS, we always want use the local resolver
+// this is necessary for being able to resolve local hostnames e.g. in a split dns setup
+// 5 means "off by choice"
+pref("network.trr.mode", 5);