--- a/dom/media/webrtc/MediaEngineWebRTC.cpp
+++ b/dom/media/webrtc/MediaEngineWebRTC.cpp
@@ -40,16 +40,17 @@ GetUserMediaLog()
 #endif
 
 #undef LOG
 #define LOG(args) MOZ_LOG(GetUserMediaLog(), mozilla::LogLevel::Debug, args)
 
 namespace mozilla {
 
 cubeb_device_collection* AudioInputCubeb::mDevices = nullptr;
+bool AudioInputCubeb::mAnyInUse = false;
 
 MediaEngineWebRTC::MediaEngineWebRTC(MediaEnginePrefs &aPrefs)
   : mMutex("mozilla::MediaEngineWebRTC"),
     mVoiceEngine(nullptr),
     mAudioInput(nullptr),
     mAudioEngineInit(false)
 {
 #ifndef MOZ_B2G_CAMERA

--- a/dom/media/webrtc/MediaEngineWebRTC.h
+++ b/dom/media/webrtc/MediaEngineWebRTC.h
@@ -154,17 +154,17 @@ protected:
 
   webrtc::VoiceEngine* mVoiceEngine;
 };
 
 class AudioInputCubeb : public AudioInput
 {
 public:
   AudioInputCubeb(webrtc::VoiceEngine* aVoiceEngine) :
-    AudioInput(aVoiceEngine), mSelectedDevice(0)
+    AudioInput(aVoiceEngine), mSelectedDevice(0), mInUse(false)
   {
     // Force calculation of the indexes.  We could keep them global
     // too... cleanup would be annoying
     int devices;
     GetNumOfRecordingDevices(devices);
   }
 
   static void CleanupGlobalData()
@@ -237,40 +237,50 @@ public:
     MOZ_ASSERT(mDevices);
 
     ScopedCustomReleasePtr<webrtc::VoEExternalMedia> ptrVoERender;
     ptrVoERender = webrtc::VoEExternalMedia::GetInterface(mVoiceEngine);
     if (ptrVoERender) {
       ptrVoERender->SetExternalRecordingStatus(true);
     }
     aGraph->OpenAudioInput(mDevices->device[mSelectedDevice]->devid, aListener);
+    mInUse = true;
+    mAnyInUse = true;
   }
 
   virtual void StopRecording(MediaStreamGraph *aGraph, AudioDataListener *aListener)
   {
     aGraph->CloseAudioInput(aListener);
+    mInUse = false;
+    mAnyInUse = false;
   }
 
   virtual int SetRecordingDevice(int aIndex)
   {
     int32_t devindex = DeviceIndex(aIndex);
-    if (!mDevices || devindex < 0) {
+    // Bug 1238038: block attempts to capture from more than one mic at a time
+    // until we can handle that in full-duplex
+    if (!mDevices || devindex < 0 || mInUse) {
       return 1;
     }
     mSelectedDevice = devindex;
     return 0;
   }
 
 protected:
-  virtual ~AudioInputCubeb() {}
+  virtual ~AudioInputCubeb() {
+    MOZ_RELEASE_ASSERT(!mInUse);
+  }
 
 private:
   nsTArray<int> mDeviceIndexes;
   int mSelectedDevice;
   static cubeb_device_collection *mDevices;
+  bool mInUse; // for assertions about listener lifetime
+  static bool mAnyInUse;
 };
 
 class AudioInputWebRTC : public AudioInput
 {
 public:
   AudioInputWebRTC(webrtc::VoiceEngine* aVoiceEngine) : AudioInput(aVoiceEngine) {}
 
   virtual int GetNumOfRecordingDevices(int& aDevices)