diff --git a/dom/base/nsGlobalWindow.cpp b/dom/base/nsGlobalWindow.cpp
index de3964e10681..28dc05ba7fd4 100644
--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -9184,6 +9184,8 @@ nsGlobalWindow::ShowModalDialog(const nsAString& aUrl, nsIVariant* aArgument,
     return nullptr;
   }
 
+  Telemetry::Accumulate(Telemetry::DOM_WINDOW_SHOWMODALDIALOG_USED, true);
+
   nsRefPtr<DialogValueHolder> argHolder =
     new DialogValueHolder(nsContentUtils::SubjectPrincipal(), aArgument);
 
diff --git a/toolkit/components/telemetry/Histograms.json b/toolkit/components/telemetry/Histograms.json
index 85e86c39695f..cd676fce15d0 100644
--- a/toolkit/components/telemetry/Histograms.json
+++ b/toolkit/components/telemetry/Histograms.json
@@ -4168,6 +4168,11 @@
     "kind": "boolean",
     "description": "DOM: Ranges that are detached on destruction (bug 702948)"
   },
+  "DOM_WINDOW_SHOWMODALDIALOG_USED": {
+    "expires_in_version": "41",
+    "kind": "flag",
+    "description": "Whether Window.showModalDialog was used in this session"
+  },
   "LOCALDOMSTORAGE_INIT_DATABASE_MS": {
     "expires_in_version": "40",
     "kind": "exponential",