--- a/browser/modules/ContentWebRTC.jsm
+++ b/browser/modules/ContentWebRTC.jsm
@@ -143,19 +143,19 @@ function handleGUMRequest(aSubject, aTop
       // devices, there's nothing to do in the callback anymore.
       if (contentWindow.closed)
         return;
 
       prompt(contentWindow, aSubject.windowID, aSubject.callID,
              constraints, devices, secure, isHandlingUserInput);
     },
     function(error) {
-      // bug 827146 -- In the future, the UI should catch NotFoundError
-      // and allow the user to plug in a device, instead of immediately failing.
-      denyGUMRequest({callID: aSubject.callID}, error);
+      // Device enumeration is done ahead of handleGUMRequest, so we're not
+      // responsible for handling the NotFoundError spec case.
+      denyGUMRequest({callID: aSubject.callID});
     },
     aSubject.innerWindowID,
     aSubject.callID);
 }
 
 function prompt(aContentWindow, aWindowID, aCallID, aConstraints, aDevices, aSecure, aIsHandlingUserInput) {
   let audioDevices = [];
   let videoDevices = [];
@@ -198,17 +198,19 @@ function prompt(aContentWindow, aWindowI
 
   let requestTypes = [];
   if (videoDevices.length)
     requestTypes.push(sharingScreen ? "Screen" : "Camera");
   if (audioDevices.length)
     requestTypes.push(sharingAudio ? "AudioCapture" : "Microphone");
 
   if (!requestTypes.length) {
-    denyGUMRequest({callID: aCallID}, "NotFoundError");
+    // Device enumeration is done ahead of handleGUMRequest, so we're not
+    // responsible for handling the NotFoundError spec case.
+    denyGUMRequest({callID: aCallID});
     return;
   }
 
   if (!aContentWindow.pendingGetUserMediaRequests) {
     setupPendingListsInitially(aContentWindow);
   }
   aContentWindow.pendingGetUserMediaRequests.set(aCallID, devices);
 
@@ -230,23 +232,18 @@ function prompt(aContentWindow, aWindowI
     audioDevices,
     videoDevices
   };
 
   let mm = getMessageManagerForWindow(aContentWindow);
   mm.sendAsyncMessage("webrtc:Request", request);
 }
 
-function denyGUMRequest(aData, aError) {
-  let msg = null;
-  if (aError) {
-    msg = Cc["@mozilla.org/supports-string;1"].createInstance(Ci.nsISupportsString);
-    msg.data = aError;
-  }
-  Services.obs.notifyObservers(msg, "getUserMedia:response:deny", aData.callID);
+function denyGUMRequest(aData) {
+  Services.obs.notifyObservers(null, "getUserMedia:response:deny", aData.callID);
 
   if (!aData.windowID)
     return;
   let contentWindow = Services.wm.getOuterWindowWithId(aData.windowID);
   if (contentWindow.pendingGetUserMediaRequests)
     forgetGUMRequest(contentWindow, aData.callID);
 }
 

--- a/dom/media/MediaManager.cpp
+++ b/dom/media/MediaManager.cpp
@@ -3660,37 +3660,21 @@ MediaManager::Observe(nsISupports* aSubj
       return task->Denied(MediaMgrError::Name::AbortError,
                           NS_LITERAL_STRING("In shutdown"));
     }
     // Reuse the same thread to save memory.
     MediaManager::PostTask(task.forget());
     return NS_OK;
 
   } else if (!strcmp(aTopic, "getUserMedia:response:deny")) {
-    MediaMgrError::Name errorName = MediaMgrError::Name::NotAllowedError;
-
-    if (aSubject) {
-      nsCOMPtr<nsISupportsString> msg(do_QueryInterface(aSubject));
-      MOZ_ASSERT(msg);
-      nsString msgData;
-      msg->GetData(msgData);
-      // The only errors other than NotAllowedError allowed by the getUserMedia
-      // spec related to selection are NotFoundError for no valid options and
-      // the catch-all AbortError for everything else (NotReadableError is
-      // reserved for device startup errors later).
-      errorName = (msgData.EqualsLiteral("NotFoundError"))
-          ? MediaMgrError::Name::NotFoundError
-          : MediaMgrError::Name::AbortError;
-    }
-
     nsString key(aData);
     RefPtr<GetUserMediaTask> task;
     mActiveCallbacks.Remove(key, getter_AddRefs(task));
     if (task) {
-      task->Denied(errorName);
+      task->Denied(MediaMgrError::Name::NotAllowedError);
       nsTArray<nsString>* array;
       if (!mCallIds.Get(task->GetWindowID(), &array)) {
         return NS_OK;
       }
       array->RemoveElement(key);
       SendPendingGUMRequest();
     }
     return NS_OK;