diff --git a/devtools/shared/DevToolsUtils.js b/devtools/shared/DevToolsUtils.js
index d44184fd6b..b9fc3660aa 100644
--- a/devtools/shared/DevToolsUtils.js
+++ b/devtools/shared/DevToolsUtils.js
@@ -537,6 +537,7 @@ function newChannelForURL(url, { policy, window, principal }) {
     // scheme to see if it helps.
     uri = Services.io.newURI("file://" + url, null, null);
   }
+
   let channelOptions = {
     contentPolicyType: policy,
     securityFlags: securityFlags,
@@ -557,15 +558,7 @@ function newChannelForURL(url, { policy, window, principal }) {
   }
   channelOptions.loadingPrincipal = prin;
 
-  try {
-    return NetUtil.newChannel(channelOptions);
-  } catch (e) {
-    // In xpcshell tests on Windows, nsExternalProtocolHandler::NewChannel()
-    // can throw NS_ERROR_UNKNOWN_PROTOCOL if the external protocol isn't
-    // supported by Windows, so we also need to handle the exception here if
-    // parsing the URL above doesn't throw.
-    return newChannelForURL("file://" + url, { policy, window, principal });
-  }
+  return NetUtil.newChannel(channelOptions);
 }
 
 // Fetch is defined differently depending on whether we are on the main thread
diff --git a/uriloader/exthandler/nsExternalProtocolHandler.cpp b/uriloader/exthandler/nsExternalProtocolHandler.cpp
index 968e109d5f..e21083357f 100644
--- a/uriloader/exthandler/nsExternalProtocolHandler.cpp
+++ b/uriloader/exthandler/nsExternalProtocolHandler.cpp
@@ -3,6 +3,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+#include "mozilla/ScopeExit.h"
 #include "nsIURI.h"
 #include "nsIURL.h"
 #include "nsExternalProtocolHandler.h"
@@ -155,21 +156,25 @@ nsresult nsExtProtocolChannel::OpenURL()
   nsresult rv = NS_ERROR_FAILURE;
   nsCOMPtr<nsIExternalProtocolService> extProtService (do_GetService(NS_EXTERNALPROTOCOLSERVICE_CONTRACTID));
 
+  auto cleanup = mozilla::MakeScopeExit([&] {
+    mCallbacks = nullptr;
+  });
+
   if (extProtService)
   {
-#ifdef DEBUG
     nsAutoCString urlScheme;
     mUrl->GetScheme(urlScheme);
     bool haveHandler = false;
     extProtService->ExternalProtocolHandlerExists(urlScheme.get(), &haveHandler);
-    NS_ASSERTION(haveHandler, "Why do we have a channel for this url if we don't support the protocol?");
-#endif
+    if (!haveHandler) {
+      return NS_ERROR_UNKNOWN_PROTOCOL;
+    }
 
     nsCOMPtr<nsIInterfaceRequestor> aggCallbacks;
     rv = NS_NewNotificationCallbacksAggregation(mCallbacks, mLoadGroup,
                                                 getter_AddRefs(aggCallbacks));
     if (NS_FAILED(rv)) {
-      goto finish;
+      return rv;
     }
                                                 
     rv = extProtService->LoadURI(mUrl, aggCallbacks);
@@ -181,8 +186,6 @@ nsresult nsExtProtocolChannel::OpenURL()
     }
   }
 
-finish:
-  mCallbacks = nullptr;
   return rv;
 }
 
@@ -476,22 +479,6 @@ nsExternalProtocolHandler::AllowPort(int32_t port, const char *scheme, bool *_re
     *_retval = false;
     return NS_OK;
 }
-// returns TRUE if the OS can handle this protocol scheme and false otherwise.
-bool nsExternalProtocolHandler::HaveExternalProtocolHandler(nsIURI * aURI)
-{
-  MOZ_ASSERT(aURI);
-  nsAutoCString scheme;
-  aURI->GetScheme(scheme);
-
-  nsCOMPtr<nsIExternalProtocolService> extProtSvc(do_GetService(NS_EXTERNALPROTOCOLSERVICE_CONTRACTID));
-  if (!extProtSvc) {
-    return false;
-  }
-
-  bool haveHandler = false;
-  extProtSvc->ExternalProtocolHandlerExists(scheme.get(), &haveHandler);
-  return haveHandler;
-}
 
 NS_IMETHODIMP nsExternalProtocolHandler::GetProtocolFlags(uint32_t *aUritype)
 {
@@ -525,14 +512,6 @@ nsExternalProtocolHandler::NewChannel2(nsIURI* aURI,
   NS_ENSURE_TRUE(aURI, NS_ERROR_UNKNOWN_PROTOCOL);
   NS_ENSURE_TRUE(aRetval, NS_ERROR_UNKNOWN_PROTOCOL);
 
-  // Only try to return a channel if we have a protocol handler for the url.
-  // nsOSHelperAppService::LoadUriInternal relies on this to check trustedness
-  // for some platforms at least.  (win uses ::ShellExecute and unix uses
-  // gnome_url_show.)
-  if (!HaveExternalProtocolHandler(aURI)) {
-    return NS_ERROR_UNKNOWN_PROTOCOL;
-  }
-
   nsCOMPtr<nsIChannel> channel = new nsExtProtocolChannel(aURI, aLoadInfo);
   channel.forget(aRetval);
   return NS_OK;
diff --git a/uriloader/exthandler/nsExternalProtocolHandler.h b/uriloader/exthandler/nsExternalProtocolHandler.h
index 426c98da18..7c0e41ad1b 100644
--- a/uriloader/exthandler/nsExternalProtocolHandler.h
+++ b/uriloader/exthandler/nsExternalProtocolHandler.h
@@ -30,8 +30,7 @@ protected:
   ~nsExternalProtocolHandler();
 
   // helper function
-  bool HaveExternalProtocolHandler(nsIURI * aURI);
-	nsCString	m_schemeName;
+  nsCString m_schemeName;
 };
 
 #endif // nsExternalProtocolHandler_h___
diff --git a/uriloader/exthandler/win/nsOSHelperAppService.h b/uriloader/exthandler/win/nsOSHelperAppService.h
index b00529433c..b38cdfdfcd 100644
--- a/uriloader/exthandler/win/nsOSHelperAppService.h
+++ b/uriloader/exthandler/win/nsOSHelperAppService.h
@@ -19,7 +19,7 @@
 #ifdef _WIN32_WINNT
 #undef _WIN32_WINNT
 #endif
-#define _WIN32_WINNT 0x0600
+#define _WIN32_WINNT 0x0601
 #include <shlobj.h>
 
 class nsMIMEInfoWin;
@@ -32,7 +32,6 @@ public:
 
   // override nsIExternalProtocolService methods
   nsresult OSProtocolHandlerExists(const char * aProtocolScheme, bool * aHandlerExists);
-  nsresult LoadUriInternal(nsIURI * aURL);
   NS_IMETHOD GetApplicationDescription(const nsACString& aScheme, nsAString& _retval);
 
   // method overrides for windows registry look up steps....