--- a/dom/media/eme/MediaKeySystemAccessManager.cpp
+++ b/dom/media/eme/MediaKeySystemAccessManager.cpp
@@ -16,16 +16,17 @@
 #endif
 #ifdef XP_MACOSX
 #include "nsCocoaFeatures.h"
 #endif
 #include "nsPrintfCString.h"
 #include "nsContentUtils.h"
 #include "nsIScriptError.h"
 #include "mozilla/Unused.h"
+#include "nsDataHashtable.h"
 
 namespace mozilla {
 namespace dom {
 
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(MediaKeySystemAccessManager)
   NS_INTERFACE_MAP_ENTRY(nsISupports)
   NS_INTERFACE_MAP_ENTRY(nsIObserver)
 NS_INTERFACE_MAP_END
@@ -164,19 +165,21 @@ MediaKeySystemAccessManager::Request(Det
     // chrome, so we can show some UI to explain how the user can rectify
     // the situation.
     MediaKeySystemAccess::NotifyObservers(mWindow, aKeySystem, status);
     aPromise->MaybeReject(NS_ERROR_DOM_NOT_SUPPORTED_ERR, message);
     return;
   }
 
   nsCOMPtr<nsIDocument> doc = mWindow->GetExtantDoc();
+  nsDataHashtable<nsCharPtrHashKey, bool> warnings;
   std::function<void(const char*)> deprecationWarningLogFn =
-    [doc](const char* aMsgName) {
+    [&](const char* aMsgName) {
       EME_LOG("Logging deprecation warning '%s' to WebConsole.", aMsgName);
+      warnings.Put(aMsgName, true);
       nsString uri;
       if (doc) {
         Unused << doc->GetDocumentURI(uri);
       }
       const char16_t* params[] = { uri.get() };
       nsContentUtils::ReportToConsole(nsIScriptError::warningFlag,
                                       NS_LITERAL_CSTRING("Media"),
                                       doc,
@@ -188,16 +191,33 @@ MediaKeySystemAccessManager::Request(Det
   MediaKeySystemConfiguration config;
   if (MediaKeySystemAccess::GetSupportedConfig(
         aKeySystem, aConfigs, config, &diagnostics, deprecationWarningLogFn)) {
     RefPtr<MediaKeySystemAccess> access(
       new MediaKeySystemAccess(mWindow, aKeySystem, config));
     aPromise->MaybeResolve(access);
     diagnostics.StoreMediaKeySystemAccess(mWindow->GetExtantDoc(),
                                           aKeySystem, true, __func__);
+
+    // Accumulate telemetry to report whether we hit deprecation warnings.
+    if (warnings.Get("MediaEMENoCapabilitiesDeprecatedWarning")) {
+      Telemetry::Accumulate(
+        Telemetry::HistogramID::MEDIA_EME_REQUEST_DEPRECATED_WARNINGS, 1);
+      EME_LOG("MEDIA_EME_REQUEST_DEPRECATED_WARNINGS "
+              "MediaEMENoCapabilitiesDeprecatedWarning");
+    } else if (warnings.Get("MediaEMENoCodecsDeprecatedWarning")) {
+      Telemetry::Accumulate(
+        Telemetry::HistogramID::MEDIA_EME_REQUEST_DEPRECATED_WARNINGS, 2);
+      EME_LOG("MEDIA_EME_REQUEST_DEPRECATED_WARNINGS "
+              "MediaEMENoCodecsDeprecatedWarning");
+    } else {
+      Telemetry::Accumulate(
+        Telemetry::HistogramID::MEDIA_EME_REQUEST_DEPRECATED_WARNINGS, 0);
+      EME_LOG("MEDIA_EME_REQUEST_DEPRECATED_WARNINGS No warnings");
+    }
     return;
   }
   // Not to inform user, because nothing to do if the corresponding keySystem
   // configuration is not supported.
   aPromise->MaybeReject(NS_ERROR_DOM_NOT_SUPPORTED_ERR,
                         NS_LITERAL_CSTRING("Key system configuration is not supported"));
   diagnostics.StoreMediaKeySystemAccess(mWindow->GetExtantDoc(),
                                         aKeySystem, false, __func__);

--- a/toolkit/components/telemetry/Histograms.json
+++ b/toolkit/components/telemetry/Histograms.json
@@ -8316,16 +8316,25 @@
     "record_in_processes": ["main", "content"],
     "alert_emails": ["cpearce@mozilla.com"],
     "bug_numbers": [1366639],
     "expires_in_version": "60",
     "kind": "enumerated",
     "n_values": 50,
     "description": "Counts of the maximum number of shared memory buffers used for transferring video frames between the CDM and Gecko processes during playback of DRM'd video. Reported once per CDMVideoDecoder instance, i.e., once per JavaScript SourceBuffer during playback of video using EME."
   },
+  "MEDIA_EME_REQUEST_DEPRECATED_WARNINGS": {
+    "record_in_processes": ["main", "content"],
+    "alert_emails": ["cpearce@mozilla.com"],
+    "bug_numbers": [1368596],
+    "expires_in_version": "60",
+    "kind": "enumerated",
+    "n_values": 10,
+    "description": "Counts types of deprecation warnings logged on every successful call to navigator.requestMediaKeySystemAccess(). 0=No warnings logged, 1=MediaEMENoCapabilitiesDeprecatedWarning, 2=MediaEMENoCodecsDeprecatedWarning."
+  },
   "MEDIACACHE_WATERMARK_KB": {
     "record_in_processes": ["main", "content"],
     "alert_emails": ["gsquelart@mozilla.com"],
     "bug_numbers": [1366929],
     "expires_in_version": "60",
     "kind": "linear",
     "high": 520000,
     "n_buckets": 66,