--- a/dom/u2f/U2F.cpp
+++ b/dom/u2f/U2F.cpp
@@ -632,38 +632,38 @@ U2FRegisterRunnable::Run()
       prepPromiseList.AppendElement(compTask->Execute());
     }
 
     // Treat each call to Promise::All as a work unit, as it completes together
     status->WaitGroupAdd();
 
     U2FPrepPromise::All(mAbstractMainThread, prepPromiseList)
     ->Then(mAbstractMainThread, __func__,
-      [status] (const nsTArray<Authenticator>& aTokens) {
+      [&status] (const nsTArray<Authenticator>& aTokens) {
         MOZ_LOG(gU2FLog, LogLevel::Debug,
                 ("ALL: None of the RegisteredKeys were recognized. n=%d",
                  aTokens.Length()));
 
         status->WaitGroupDone();
       },
-      [status] (ErrorCode aErrorCode) {
+      [&status] (ErrorCode aErrorCode) {
         status->Stop(aErrorCode);
         status->WaitGroupDone();
     });
   }
 
   // Wait for all the IsRegistered tasks to complete
   status->WaitGroupWait();
 
   // Check to see whether we're supposed to stop, because one of the keys was
   // recognized.
   if (status->IsStopped()) {
     status->WaitGroupAdd();
     mAbstractMainThread->Dispatch(NS_NewRunnableFunction(
-      [status, this] () {
+      [&status, this] () {
         RegisterResponse response;
         response.mErrorCode.Construct(
             static_cast<uint32_t>(status->GetErrorCode()));
         SendResponse(response);
         status->WaitGroupDone();
       }
     ));
 
@@ -708,45 +708,43 @@ U2FRegisterRunnable::Run()
       RefPtr<U2FRegisterTask> registerTask = new U2FRegisterTask(mOrigin, mAppId,
                                                                  token, appParam,
                                                                  challengeParam,
                                                                  req,
                                                                  mAbstractMainThread);
       status->WaitGroupAdd();
 
       registerTask->Execute()->Then(mAbstractMainThread, __func__,
-        [status, this] (nsString aResponse) {
-          if (status->IsStopped()) {
-            return;
+        [&status, this] (nsString aResponse) {
+          if (!status->IsStopped()) {
+            status->Stop(ErrorCode::OK, aResponse);
           }
-          status->Stop(ErrorCode::OK, aResponse);
           status->WaitGroupDone();
         },
-        [status, this] (ErrorCode aErrorCode) {
-          if (status->IsStopped()) {
-            return;
+        [&status, this] (ErrorCode aErrorCode) {
+          if (!status->IsStopped()) {
+            status->Stop(aErrorCode);
           }
-          status->Stop(aErrorCode);
           status->WaitGroupDone();
      });
     }
   }
 
   // Wait until the first key is successfuly generated
   status->WaitGroupWait();
 
   // If none of the tasks completed, then nothing could satisfy.
   if (!status->IsStopped()) {
     status->Stop(ErrorCode::BAD_REQUEST);
   }
 
   // Transmit back to the JS engine from the Main Thread
   status->WaitGroupAdd();
   mAbstractMainThread->Dispatch(NS_NewRunnableFunction(
-    [status, this] () {
+    [&status, this] () {
       RegisterResponse response;
       if (status->GetErrorCode() == ErrorCode::OK) {
         response.Init(status->GetResponse());
       } else {
         response.mErrorCode.Construct(
             static_cast<uint32_t>(status->GetErrorCode()));
       }
       SendResponse(response);
@@ -896,45 +894,43 @@ U2FSignRunnable::Run()
       RefPtr<U2FSignTask> signTask = new U2FSignTask(mOrigin, mAppId,
                                                      key.mVersion, token,
                                                      appParam, challengeParam,
                                                      mClientData, keyHandle,
                                                      mAbstractMainThread);
       status->WaitGroupAdd();
 
       signTask->Execute()->Then(mAbstractMainThread, __func__,
-        [status, this] (nsString aResponse) {
-          if (status->IsStopped()) {
-            return;
+        [&status, this] (nsString aResponse) {
+          if (!status->IsStopped()) {
+            status->Stop(ErrorCode::OK, aResponse);
           }
-          status->Stop(ErrorCode::OK, aResponse);
           status->WaitGroupDone();
         },
-        [status, this] (ErrorCode aErrorCode) {
-          if (status->IsStopped()) {
-            return;
+        [&status, this] (ErrorCode aErrorCode) {
+          if (!status->IsStopped()) {
+            status->Stop(aErrorCode);
           }
-          status->Stop(aErrorCode);
           status->WaitGroupDone();
       });
     }
   }
 
   // Wait for the authenticators to finish
   status->WaitGroupWait();
 
   // If none of the tasks completed, then nothing could satisfy.
   if (!status->IsStopped()) {
     status->Stop(ErrorCode::DEVICE_INELIGIBLE);
   }
 
   // Transmit back to the JS engine from the Main Thread
   status->WaitGroupAdd();
   mAbstractMainThread->Dispatch(NS_NewRunnableFunction(
-    [status, this] () {
+    [&status, this] () {
       SignResponse response;
       if (status->GetErrorCode() == ErrorCode::OK) {
         response.Init(status->GetResponse());
       } else {
         response.mErrorCode.Construct(
           static_cast<uint32_t>(status->GetErrorCode()));
       }
       SendResponse(response);

--- a/dom/u2f/U2F.h
+++ b/dom/u2f/U2F.h
@@ -165,16 +165,17 @@ private:
 };
 
 // Mediate inter-thread communication for multiple authenticators being queried
 // in concert. Operates as a cyclic buffer with a stop-work method.
 class U2FStatus
 {
 public:
   U2FStatus();
+  U2FStatus(const U2FStatus&) = delete;
 
   void WaitGroupAdd();
   void WaitGroupDone();
   void WaitGroupWait();
 
   void Stop(const ErrorCode aErrorCode);
   void Stop(const ErrorCode aErrorCode, const nsAString& aResponse);
   bool IsStopped();

new file mode 100644
--- /dev/null
+++ b/dom/u2f/tests/README.md
@@ -0,0 +1,8 @@
+Note:
+
+While conceptually similar to the tests for Web Authentication (dom/webauthn),
+the tests for U2F require an iframe while `window.u2f` remains hidden behind a
+preference, though WebAuthn does not. The reason is that the `window` object
+doesn't mutate upon a call by SpecialPowers.setPrefEnv() the way that the
+`navigator` objects do, rather you have to load a different page with a different
+`window` object for the preference change to be honored.

new file mode 100644
--- /dev/null
+++ b/dom/u2f/tests/frame_multiple_keys.html
@@ -0,0 +1,83 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<head>
+  <script type="text/javascript" src="u2futil.js"></script>
+</head>
+<body>
+
+<script class="testbody" type="text/javascript">
+"use strict";
+
+function keyHandleFromRegResponse(aRegResponse) {
+  // Parse the response data from the U2F token
+  var registrationData = base64ToBytesUrlSafe(aRegResponse.registrationData);
+  local_is(registrationData[0], 0x05, "Reserved byte is correct")
+
+  var keyHandleLength = registrationData[66];
+  var keyHandleBytes = registrationData.slice(67, 67 + keyHandleLength);
+
+  return {
+    version: "U2F_V2",
+    keyHandle: bytesToBase64UrlSafe(keyHandleBytes),
+  };
+}
+
+local_setParentOrigin("https://example.com");
+local_expectThisManyTests(1);
+
+// Ensure the SpecialPowers push worked properly
+local_isnot(window.u2f, undefined, "U2F API endpoint must exist");
+
+var challenge = new Uint8Array(16);
+window.crypto.getRandomValues(challenge);
+
+var regRequest = {
+  version: "U2F_V2",
+  challenge: bytesToBase64UrlSafe(challenge),
+};
+
+var testState = {
+  key1: null,
+  key2: null,
+}
+
+// Get two valid keys and present them
+window.u2f.register(window.location.origin, [regRequest], [], function(aRegResponse) {
+  testState.key1 = keyHandleFromRegResponse(aRegResponse);
+  registerSecondKey();
+});
+
+// Get the second key...
+// It's OK to repeat the regRequest; not material for this test
+function registerSecondKey() {
+  window.u2f.register(window.location.origin, [regRequest], [], function(aRegResponse) {
+    testState.key2 = keyHandleFromRegResponse(aRegResponse);
+    testSignSingleKey();
+  });
+}
+
+// It should also work with just one key
+function testSignSingleKey() {
+  window.u2f.sign(window.location.origin, bytesToBase64UrlSafe(challenge),
+                  [testState.key1], function(aSignResponse) {
+    local_is(aSignResponse.errorCode, 0, "The signing did not error with one key");
+    local_isnot(aSignResponse.clientData, undefined, "The signing provided clientData with one key");
+
+    testSignDual();
+  });
+}
+
+function testSignDual() {
+  // It's OK to sign with either one
+  window.u2f.sign(window.location.origin, bytesToBase64UrlSafe(challenge),
+                  [testState.key1, testState.key2], function(aSignResponse) {
+    local_is(aSignResponse.errorCode, 0, "The signing did not error with two keys");
+    local_isnot(aSignResponse.clientData, undefined, "The signing provided clientData with two keys");
+
+    local_completeTest();
+  });
+}
+</script>
+
+</body>
+</html>
\ No newline at end of file

--- a/dom/u2f/tests/mochitest.ini
+++ b/dom/u2f/tests/mochitest.ini
@@ -1,16 +1,17 @@
 [DEFAULT]
 support-files =
   frame_appid_facet.html
   frame_appid_facet_insecure.html
   frame_appid_facet_subdomain.html
   frame_no_token.html
   frame_register.html
   frame_register_sign.html
+  frame_multiple_keys.html
   pkijs/asn1.js
   pkijs/common.js
   pkijs/x509_schema.js
   pkijs/x509_simpl.js
   u2futil.js
 
 # Feature does not function on e10s (Disabled in Bug 1297552)
 [test_util_methods.html]
@@ -22,8 +23,11 @@ skip-if = !e10s
 [test_register_sign.html]
 skip-if = !e10s
 [test_appid_facet.html]
 skip-if = !e10s
 [test_appid_facet_insecure.html]
 skip-if = !e10s
 [test_appid_facet_subdomain.html]
 skip-if = !e10s
+[test_multiple_keys.html]
+skip-if = !e10s
+scheme = https

new file mode 100644
--- /dev/null
+++ b/dom/u2f/tests/test_multiple_keys.html
@@ -0,0 +1,39 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<head>
+  <title>Test for Multiple Keys for FIDO U2F</title>
+  <script type="text/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
+  <script src="u2futil.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css" />
+</head>
+<body>
+
+<h1>Test Multiple Keys for FIDO U2F</h1>
+<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1333592">Mozilla Bug 1333592</a>
+
+<script class="testbody" type="text/javascript">
+"use strict";
+
+ /** Test for XBL scope behavior. **/
+SimpleTest.waitForExplicitFinish();
+
+// Embed the real test. It will take care of everything else.
+//
+// This is necessary since the U2F object on window is hidden behind a preference
+// and window won't pick up changes by pref without a reload.
+SpecialPowers.pushPrefEnv({"set": [["security.webauth.u2f", true],
+                                   ["security.webauth.u2f_enable_softtoken", true],
+                                   ["security.webauth.u2f_enable_usbtoken", false]]},
+function() {
+  // listen for messages from the test harness
+  window.addEventListener("message", handleEventMessage);
+  document.getElementById('testing_frame').src = "frame_multiple_keys.html";
+});
+</script>
+
+<div id="framediv">
+  <iframe id="testing_frame"></iframe>
+</div>
+
+</body>
+</html>

--- a/dom/u2f/tests/u2futil.js
+++ b/dom/u2f/tests/u2futil.js
@@ -1,11 +1,16 @@
 // Used by local_addTest() / local_completeTest()
 var _countCompletions = 0;
 var _expectedCompletions = 0;
+var _parentOrigin = "http://mochi.test:8888";
+
+function local_setParentOrigin(aOrigin) {
+  _parentOrigin = aOrigin;
+}
 
 function handleEventMessage(event) {
   if ("test" in event.data) {
     let summary = event.data.test + ": " + event.data.msg;
     log(event.data.status + ": " + summary);
     ok(event.data.status, summary);
   } else if ("done" in event.data) {
     SimpleTest.finish();
@@ -35,17 +40,17 @@ function local_isnot(value, expected, me
     local_ok(true, message);
   } else {
     local_ok(false, message + " unexpectedly: " + value + " === " + expected);
   }
 }
 
 function local_ok(expression, message) {
   let body = {"test": this.location.pathname, "status":expression, "msg": message}
-  parent.postMessage(body, "http://mochi.test:8888");
+  parent.postMessage(body, _parentOrigin);
 }
 
 function local_doesThrow(fn, name) {
   let gotException = false;
   try {
     fn();
   } catch (ex) { gotException = true; }
   local_ok(gotException, name);
@@ -65,17 +70,17 @@ function local_completeTest() {
     local_finished();
   }
   if (_countCompletions > _expectedCompletions) {
     local_ok(false, "Error: local_completeTest called more than local_addTest.");
   }
 }
 
 function local_finished() {
-  parent.postMessage({"done":true}, "http://mochi.test:8888");
+  parent.postMessage({"done":true}, _parentOrigin);
 }
 
 function string2buffer(str) {
   return (new Uint8Array(str.length)).map((x, i) => str.charCodeAt(i));
 }
 
 function buffer2string(buf) {
   let str = "";