diff --git a/browser/components/BrowserGlue.jsm b/browser/components/BrowserGlue.jsm
index 97172158a09a..fbcc9e06c3c7 100644
--- a/browser/components/BrowserGlue.jsm
+++ b/browser/components/BrowserGlue.jsm
@@ -82,6 +82,7 @@ XPCOMUtils.defineLazyModuleGetters(this, {
   SearchSERPTelemetry: "resource:///modules/SearchSERPTelemetry.jsm",
   SessionStartup: "resource:///modules/sessionstore/SessionStartup.jsm",
   SessionStore: "resource:///modules/sessionstore/SessionStore.jsm",
+  ShellService: "resource:///modules/ShellService.jsm",
   ShortcutUtils: "resource://gre/modules/ShortcutUtils.jsm",
   TabCrashHandler: "resource:///modules/ContentCrashHandlers.jsm",
   TabUnloader: "resource:///modules/TabUnloader.jsm",
@@ -2465,6 +2466,19 @@ BrowserGlue.prototype = {
         },
       },
 
+      // Report whether Firefox is the default handler for various files types,
+      // in particular, ".pdf".
+      {
+        condition: AppConstants.platform == "win",
+        task: () => {
+          Services.telemetry.keyedScalarSet(
+            "os.environment.is_default_handler",
+            ".pdf",
+            ShellService.isDefaultHandlerFor(".pdf")
+          );
+        },
+      },
+
       // Install built-in themes. We already installed the active built-in
       // theme, if any, before UI startup.
       {
diff --git a/browser/components/shell/ShellService.jsm b/browser/components/shell/ShellService.jsm
index af24c65f931f..2ad325e737b2 100644
--- a/browser/components/shell/ShellService.jsm
+++ b/browser/components/shell/ShellService.jsm
@@ -289,6 +289,21 @@ let ShellServiceInternal = {
       .add(setAsDefaultError);
   },
 
+  /**
+   * Determine if we're the default handler for the given file extension (like
+   * ".pdf") or protocol (like "https").  Windows-only for now.
+   *
+   * @returns true if we are the default handler, false otherwise.
+   */
+  isDefaultHandlerFor(aFileExtensionOrProtocol) {
+    if (AppConstants.platform == "win") {
+      return this.shellService
+        .QueryInterface(Ci.nsIWindowsShellService)
+        .isDefaultHandlerFor(aFileExtensionOrProtocol);
+    }
+    return false;
+  },
+
   /**
    * Checks if Firefox app can and isn't pinned to OS "taskbar."
    *
diff --git a/browser/components/shell/nsIWindowsShellService.idl b/browser/components/shell/nsIWindowsShellService.idl
index 9dee21a5d452..c08fb6854fcd 100644
--- a/browser/components/shell/nsIWindowsShellService.idl
+++ b/browser/components/shell/nsIWindowsShellService.idl
@@ -108,4 +108,11 @@ interface nsIWindowsShellService : nsISupports
    */
   bool checkAllProgIDsExist();
   bool checkBrowserUserChoiceHashes();
+
+  /*
+   * Determines whether or not Firefox is the "Default Handler", i.e.,
+   * is registered to handle, the given file extension (like ".pdf")
+   * or protocol (like "https").
+   */
+  boolean isDefaultHandlerFor(in AString aFileExtensionOrProtocol);
 };
diff --git a/browser/components/shell/nsWindowsShellService.cpp b/browser/components/shell/nsWindowsShellService.cpp
index 53e15984e0da..e279a75fa6f8 100644
--- a/browser/components/shell/nsWindowsShellService.cpp
+++ b/browser/components/shell/nsWindowsShellService.cpp
@@ -207,6 +207,32 @@ nsWindowsShellService::IsDefaultBrowser(bool aForAllTypes,
   return NS_OK;
 }
 
+NS_IMETHODIMP
+nsWindowsShellService::IsDefaultHandlerFor(
+    const nsAString& aFileExtensionOrProtocol, bool* aIsDefaultHandlerFor) {
+  *aIsDefaultHandlerFor = false;
+
+  RefPtr<IApplicationAssociationRegistration> pAAR;
+  HRESULT hr = CoCreateInstance(
+      CLSID_ApplicationAssociationRegistration, nullptr, CLSCTX_INPROC,
+      IID_IApplicationAssociationRegistration, getter_AddRefs(pAAR));
+  if (FAILED(hr)) {
+    return NS_OK;
+  }
+
+  wchar_t exePath[MAXPATHLEN] = L"";
+  nsresult rv = BinaryPath::GetLong(exePath);
+
+  if (NS_FAILED(rv)) {
+    return NS_OK;
+  }
+
+  const nsString& flatClass = PromiseFlatString(aFileExtensionOrProtocol);
+
+  *aIsDefaultHandlerFor = IsPathDefaultForClass(pAAR, exePath, flatClass.get());
+  return NS_OK;
+}
+
 nsresult nsWindowsShellService::LaunchControlPanelDefaultsSelectionUI() {
   IApplicationAssociationRegistrationUI* pAARUI;
   HRESULT hr = CoCreateInstance(
diff --git a/browser/components/shell/test/browser_setDefaultPDFHandler.js b/browser/components/shell/test/browser_setDefaultPDFHandler.js
index 244113b03aaa..9506e83f1ede 100644
--- a/browser/components/shell/test/browser_setDefaultPDFHandler.js
+++ b/browser/components/shell/test/browser_setDefaultPDFHandler.js
@@ -49,16 +49,12 @@ if (AppConstants.isPlatformAndVersionAtLeast("win", "10")) {
   // Everything here is Windows 10+.
 
   add_task(async function remoteEnableWithPDF() {
-    await ExperimentFakes.remoteDefaultsHelper({
-      feature: NimbusFeatures.shellService,
-      configuration: {
-        slug: "shellService_remoteEnableWithPDF",
-        variables: {
-          setDefaultBrowserUserChoice: true,
-          setDefaultPDFHandler: true,
-          enabled: true,
-        },
-        targeting: "true",
+    let doCleanup = await ExperimentFakes.enrollWithRollout({
+      featureId: NimbusFeatures.shellService.featureId,
+      value: {
+        setDefaultBrowserUserChoice: true,
+        setDefaultPDFHandler: true,
+        enabled: true,
       },
     });
 
@@ -79,19 +75,17 @@ if (AppConstants.isPlatformAndVersionAtLeast("win", "10")) {
     Assert.deepEqual(_callExternalDefaultBrowserAgentStub.firstCall.args, [
       { arguments: ["set-default-browser-user-choice", aumi, ".pdf"] },
     ]);
+
+    await doCleanup();
   });
 
   add_task(async function remoteEnableWithoutPDF() {
-    await ExperimentFakes.remoteDefaultsHelper({
-      feature: NimbusFeatures.shellService,
-      configuration: {
-        slug: "shellService_remoteEnableWithoutPDF",
-        variables: {
-          setDefaultBrowserUserChoice: true,
-          setDefaultPDFHandler: false,
-          enabled: true,
-        },
-        targeting: "true",
+    let doCleanup = await ExperimentFakes.enrollWithRollout({
+      featureId: NimbusFeatures.shellService.featureId,
+      value: {
+        setDefaultBrowserUserChoice: true,
+        setDefaultPDFHandler: false,
+        enabled: true,
       },
     });
 
@@ -112,19 +106,17 @@ if (AppConstants.isPlatformAndVersionAtLeast("win", "10")) {
     Assert.deepEqual(_callExternalDefaultBrowserAgentStub.firstCall.args, [
       { arguments: ["set-default-browser-user-choice", aumi] },
     ]);
+
+    await doCleanup();
   });
 
   add_task(async function remoteDisable() {
-    await ExperimentFakes.remoteDefaultsHelper({
-      feature: NimbusFeatures.shellService,
-      configuration: {
-        slug: "shellService_remoteDisable",
-        variables: {
-          setDefaultBrowserUserChoice: false,
-          setDefaultPDFHandler: true,
-          enabled: false,
-        },
-        targeting: "true",
+    let doCleanup = await ExperimentFakes.enrollWithRollout({
+      featureId: NimbusFeatures.shellService.featureId,
+      value: {
+        setDefaultBrowserUserChoice: false,
+        setDefaultPDFHandler: true,
+        enabled: false,
       },
     });
 
@@ -142,5 +134,7 @@ if (AppConstants.isPlatformAndVersionAtLeast("win", "10")) {
 
     Assert.ok(_callExternalDefaultBrowserAgentStub.notCalled);
     Assert.ok(setDefaultStub.called);
+
+    await doCleanup();
   });
 }
diff --git a/browser/components/tests/browser/browser_browserGlue_telemetry.js b/browser/components/tests/browser/browser_browserGlue_telemetry.js
index 6ee9dbacbd40..4ff4c8c4561c 100644
--- a/browser/components/tests/browser/browser_browserGlue_telemetry.js
+++ b/browser/components/tests/browser/browser_browserGlue_telemetry.js
@@ -45,3 +45,42 @@ add_task(function check_startup_pinned_telemetry() {
       break;
   }
 });
+
+// Check that telemetry reports whether Firefox is the default PDF handler.
+// This is safe without any explicit coordination because idle tasks are
+// guaranteed to have been invokedbefore the test harness invokes the test.  See
+// https://searchfox.org/mozilla-central/rev/1674b86019a96f076e0f98f1d0f5f3ab9d4e9020/browser/components/BrowserGlue.jsm#2320-2324
+// and
+// https://searchfox.org/mozilla-central/rev/1674b86019a96f076e0f98f1d0f5f3ab9d4e9020/browser/base/content/browser.js#2364.
+add_task(function check_is_default_handler_telemetry() {
+  const scalars = TelemetryTestUtils.getProcessScalars("parent", true);
+
+  // Check the appropriate telemetry is set or not reported by platform.
+  switch (AppConstants.platform) {
+    case "win":
+      // We should always set whether we're the default PDF handler.
+      Assert.ok("os.environment.is_default_handler" in scalars);
+      Assert.deepEqual(
+        [".pdf"],
+        Object.keys(scalars["os.environment.is_default_handler"])
+      );
+
+      if (Cu.isInAutomation) {
+        // But only in automation can we assume we're not the default handler.
+        TelemetryTestUtils.assertKeyedScalar(
+          scalars,
+          "os.environment.is_default_handler",
+          ".pdf",
+          false,
+          "Not default PDF handler on Windows"
+        );
+      }
+      break;
+    default:
+      TelemetryTestUtils.assertScalarUnset(
+        scalars,
+        "os.environment.is_default_handler"
+      );
+      break;
+  }
+});
diff --git a/toolkit/components/telemetry/Scalars.yaml b/toolkit/components/telemetry/Scalars.yaml
index 9b934850fd54..89f2035e75a7 100644
--- a/toolkit/components/telemetry/Scalars.yaml
+++ b/toolkit/components/telemetry/Scalars.yaml
@@ -1922,6 +1922,29 @@ os.environment:
     operating_systems:
       - windows
 
+  is_default_handler:
+    bug_numbers:
+      - 1743914
+    description: >
+      Records whether Firefox was the default handler for particular file types
+      or protocols.  The result is split into keys which represent the file
+      extension: currently, a subset of the file types Firefox registers to
+      handle, namely ".pdf".  In the future, more file types may be recorded,
+      and/or a subset of the protocol schemes that Firefox registers to may be
+      recorded.
+    keyed: true
+    expires: "106"
+    kind: boolean
+    notification_emails:
+      - application-update-telemetry-alerts@mozilla.com
+    release_channel_collection: opt-out
+    products:
+      - firefox
+    record_in_processes:
+      - main
+    operating_systems:
+      - windows
+
   is_kept_in_dock:
     bug_numbers:
       - 1715348