diff --git a/dom/media/MediaManager.cpp b/dom/media/MediaManager.cpp
index 7e3ed762360d..a06296086a28 100644
--- a/dom/media/MediaManager.cpp
+++ b/dom/media/MediaManager.cpp
@@ -1232,8 +1232,13 @@ public:
                                                  self->mSourceListener->GetPrincipalHandle());
         if (NS_FAILED(rv)) {
           nsString log;
-          log.AssignASCII("Starting audio failed");
-          error = new MediaMgrError(NS_LITERAL_STRING("InternalError"), log);
+          if (rv == NS_ERROR_NOT_AVAILABLE) {
+            log.AssignASCII("Concurrent mic process limit.");
+            error = new MediaMgrError(NS_LITERAL_STRING("NotReadableError"), log);
+          } else {
+            log.AssignASCII("Starting audio failed");
+            error = new MediaMgrError(NS_LITERAL_STRING("InternalError"), log);
+          }
         }
       }
 
diff --git a/dom/media/webrtc/MediaEngineWebRTCAudio.cpp b/dom/media/webrtc/MediaEngineWebRTCAudio.cpp
index 3f3b6074fd4b..ff77065b09fa 100644
--- a/dom/media/webrtc/MediaEngineWebRTCAudio.cpp
+++ b/dom/media/webrtc/MediaEngineWebRTCAudio.cpp
@@ -474,6 +474,13 @@ MediaEngineWebRTCMicrophoneSource::Start(SourceMediaStream *aStream,
     return NS_ERROR_FAILURE;
   }
 
+  // Until we fix bug 1400488 we need to block a second tab (OuterWindow)
+  // from opening an already-open device.  If it's the same tab, they
+  // will share a Graph(), and we can allow it.
+  if (!mSources.IsEmpty() && aStream->Graph() != mSources[0]->Graph()) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
   {
     MonitorAutoLock lock(mMonitor);
     mSources.AppendElement(aStream);