--- a/toolkit/components/telemetry/TelemetryEnvironment.jsm
+++ b/toolkit/components/telemetry/TelemetryEnvironment.jsm
@@ -143,16 +143,17 @@ this.TelemetryEnvironment = {
   shutdown() {
     return getGlobal().shutdown();
   },
 
   // Policy to use when saving preferences. Exported for using them in tests.
   RECORD_PREF_STATE: 1, // Don't record the preference value
   RECORD_PREF_VALUE: 2, // We only record user-set prefs.
   RECORD_DEFAULTPREF_VALUE: 3, // We only record default pref if set
+  RECORD_DEFAULTPREF_STATE: 4, // We only record if the pref exists
 
   // Testing method
   testWatchPreferences(prefMap) {
     return getGlobal()._watchPreferences(prefMap);
   },
 
   /**
    * Intended for use in tests only.
@@ -175,16 +176,17 @@ this.TelemetryEnvironment = {
     gActiveExperimentStartupBuffer = new Map();
     return getGlobal();
   },
 };
 
 const RECORD_PREF_STATE = TelemetryEnvironment.RECORD_PREF_STATE;
 const RECORD_PREF_VALUE = TelemetryEnvironment.RECORD_PREF_VALUE;
 const RECORD_DEFAULTPREF_VALUE = TelemetryEnvironment.RECORD_DEFAULTPREF_VALUE;
+const RECORD_DEFAULTPREF_STATE = TelemetryEnvironment.RECORD_DEFAULTPREF_STATE;
 const DEFAULT_ENVIRONMENT_PREFS = new Map([
   ["app.feedback.baseURL", {what: RECORD_PREF_VALUE}],
   ["app.support.baseURL", {what: RECORD_PREF_VALUE}],
   ["accessibility.browsewithcaret", {what: RECORD_PREF_VALUE}],
   ["accessibility.force_disabled", {what:  RECORD_PREF_VALUE}],
   ["app.update.auto", {what: RECORD_PREF_VALUE}],
   ["app.update.enabled", {what: RECORD_PREF_VALUE}],
   ["app.update.interval", {what: RECORD_PREF_VALUE}],
@@ -223,16 +225,17 @@ const DEFAULT_ENVIRONMENT_PREFS = new Ma
   ["extensions.formautofill.addresses.enabled", {what: RECORD_PREF_VALUE}],
   ["extensions.formautofill.creditCards.enabled", {what: RECORD_PREF_VALUE}],
   ["extensions.legacy.enabled", {what: RECORD_PREF_VALUE}],
   ["extensions.strictCompatibility", {what: RECORD_PREF_VALUE}],
   ["extensions.update.enabled", {what: RECORD_PREF_VALUE}],
   ["extensions.update.url", {what: RECORD_PREF_VALUE}],
   ["extensions.update.background.url", {what: RECORD_PREF_VALUE}],
   ["extensions.screenshots.disabled", {what: RECORD_PREF_VALUE}],
+  ["general.config.filename", {what: RECORD_DEFAULTPREF_STATE}],
   ["general.smoothScroll", {what: RECORD_PREF_VALUE}],
   ["gfx.direct2d.disabled", {what: RECORD_PREF_VALUE}],
   ["gfx.direct2d.force-enabled", {what: RECORD_PREF_VALUE}],
   ["layers.acceleration.disabled", {what: RECORD_PREF_VALUE}],
   ["layers.acceleration.force-enabled", {what: RECORD_PREF_VALUE}],
   ["layers.async-pan-zoom.enabled", {what: RECORD_PREF_VALUE}],
   ["layers.async-video-oop.enabled", {what: RECORD_PREF_VALUE}],
   ["layers.async-video.enabled", {what: RECORD_PREF_VALUE}],
@@ -1028,30 +1031,34 @@ EnvironmentCache.prototype = {
    *
    * @return An object containing the preferences values.
    */
   _getPrefData() {
     let prefData = {};
     for (let [pref, policy] of this._watchedPrefs.entries()) {
       let prefType = Services.prefs.getPrefType(pref);
 
-      if (policy.what == TelemetryEnvironment.RECORD_DEFAULTPREF_VALUE) {
+      let what = policy.what;
+      if (what == TelemetryEnvironment.RECORD_DEFAULTPREF_VALUE ||
+          what == TelemetryEnvironment.RECORD_DEFAULTPREF_STATE) {
         // For default prefs, make sure they exist
         if (prefType == Ci.nsIPrefBranch.PREF_INVALID) {
           continue;
         }
       } else if (!Services.prefs.prefHasUserValue(pref)) {
         // For user prefs, make sure they are set
         continue;
       }
 
       // Check the policy for the preference and decide if we need to store its value
       // or whether it changed from the default value.
       let prefValue;
-      if (policy.what == TelemetryEnvironment.RECORD_PREF_STATE) {
+      if (what == TelemetryEnvironment.RECORD_DEFAULTPREF_STATE) {
+        prefValue = "<set>";
+      } else if (what == TelemetryEnvironment.RECORD_PREF_STATE) {
         prefValue = "<user-set>";
       } else if (prefType == Ci.nsIPrefBranch.PREF_STRING) {
         prefValue = Services.prefs.getStringPref(pref);
       } else if (prefType == Ci.nsIPrefBranch.PREF_BOOL) {
         prefValue = Services.prefs.getBoolPref(pref);
       } else if (prefType == Ci.nsIPrefBranch.PREF_INT) {
         prefValue = Services.prefs.getIntPref(pref);
       } else if (prefType == Ci.nsIPrefBranch.PREF_INVALID) {

--- a/toolkit/components/telemetry/tests/unit/test_TelemetryEnvironment.js
+++ b/toolkit/components/telemetry/tests/unit/test_TelemetryEnvironment.js
@@ -1026,16 +1026,34 @@ add_task(async function test_prefDefault
 
   // Set the Environment preferences to watch.
   // We're not watching, but this function does the setup we need.
   TelemetryEnvironment.testWatchPreferences(PREFS_TO_WATCH);
 
   Assert.strictEqual(TelemetryEnvironment.currentEnvironment.settings.userPrefs[PREF_TEST], expectedValue);
 });
 
+add_task(async function test_prefDefaultState() {
+  const PREF_TEST = "toolkit.telemetry.test.defaultpref2";
+  const expectedValue = "some-test-value";
+
+  const PREFS_TO_WATCH = new Map([
+    [PREF_TEST, {what: TelemetryEnvironment.RECORD_DEFAULTPREF_STATE}],
+  ]);
+
+  TelemetryEnvironment.testWatchPreferences(PREFS_TO_WATCH);
+
+  Assert.equal(PREF_TEST in TelemetryEnvironment.currentEnvironment.settings.userPrefs, false);
+
+  // Set the preference to a default value.
+  Services.prefs.getDefaultBranch(null).setCharPref(PREF_TEST, expectedValue);
+
+  Assert.strictEqual(TelemetryEnvironment.currentEnvironment.settings.userPrefs[PREF_TEST], "<set>");
+});
+
 add_task(async function test_addonsWatch_InterestingChange() {
   const ADDON_INSTALL_URL = gDataRoot + "restartless.xpi";
   const ADDON_ID = "tel-restartless-webext@tests.mozilla.org";
   // We only expect a single notification for each install, uninstall, enable, disable.
   const EXPECTED_NOTIFICATIONS = 4;
 
   let receivedNotifications = 0;