diff --git a/dom/base/nsGlobalWindow.cpp b/dom/base/nsGlobalWindow.cpp
index 97b1122ba65b..366c902988ae 100644
--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -1009,6 +1009,9 @@ nsGlobalWindow::~nsGlobalWindow()
       PR_REMOVE_AND_INIT_LINK(w);
     }
   } else {
+    Telemetry::Accumulate(Telemetry::INNERWINDOWS_WITH_MUTATION_LISTENERS,
+                          mMutationBits ? 1 : 0);
+
     if (mListenerManager) {
       mListenerManager->Disconnect();
       mListenerManager = nsnull;
@@ -2365,6 +2368,9 @@ nsGlobalWindow::InnerSetNewDocument(nsIDocument* aDocument)
   mLastOpenedURI = aDocument->GetDocumentURI();
 #endif
 
+  Telemetry::Accumulate(Telemetry::INNERWINDOWS_WITH_MUTATION_LISTENERS,
+                        mMutationBits ? 1 : 0);
+
   // Clear our mutation bitfield.
   mMutationBits = 0;
 
diff --git a/toolkit/components/telemetry/TelemetryHistograms.h b/toolkit/components/telemetry/TelemetryHistograms.h
index 3ab8e6e57fe9..601ddd498c6f 100644
--- a/toolkit/components/telemetry/TelemetryHistograms.h
+++ b/toolkit/components/telemetry/TelemetryHistograms.h
@@ -233,3 +233,5 @@ HISTOGRAM(THUNDERBIRD_GLODA_SIZE_MB, 1, 1000, 40, LINEAR, "Gloda: size of global
 HISTOGRAM(THUNDERBIRD_CONVERSATIONS_TIME_TO_2ND_GLODA_QUERY_MS, 1, 10000, 30, EXPONENTIAL, "Conversations: time between the moment we click and the second gloda query returns (ms)")
 HISTOGRAM(THUNDERBIRD_INDEXING_RATE_MSG_PER_S, 1, 100, 20, LINEAR, "Gloda: indexing rate (message/s)")
 #endif
+
+HISTOGRAM(INNERWINDOWS_WITH_MUTATION_LISTENERS, 0, 1, 2, BOOLEAN, "Deleted or to-be-reused innerwindow which has had mutation event listeners.")