--- a/netwerk/protocol/res/nsResProtocolHandler.cpp
+++ b/netwerk/protocol/res/nsResProtocolHandler.cpp
@@ -16,56 +16,37 @@
 #include "nsEscape.h"
 
 #include "mozilla/Omnijar.h"
 
 using mozilla::dom::ContentParent;
 using mozilla::LogLevel;
 using mozilla::Unused;
 
-#define kAPP           NS_LITERAL_CSTRING("app")
-#define kGRE           NS_LITERAL_CSTRING("gre")
+#define kAPP           "app"
+#define kGRE           "gre"
 
 nsresult
 nsResProtocolHandler::Init()
 {
     nsresult rv;
-    nsAutoCString appURI, greURI;
-    rv = mozilla::Omnijar::GetURIString(mozilla::Omnijar::APP, appURI);
-    NS_ENSURE_SUCCESS(rv, rv);
-    rv = mozilla::Omnijar::GetURIString(mozilla::Omnijar::GRE, greURI);
+    rv = mozilla::Omnijar::GetURIString(mozilla::Omnijar::APP, mAppURI);
     NS_ENSURE_SUCCESS(rv, rv);
-
-    //
-    // make resource:/// point to the application directory or omnijar
-    //
-    nsCOMPtr<nsIURI> uri;
-    rv = NS_NewURI(getter_AddRefs(uri), appURI.Length() ? appURI : greURI);
-    NS_ENSURE_SUCCESS(rv, rv);
-
-    rv = SetSubstitution(EmptyCString(), uri);
+    rv = mozilla::Omnijar::GetURIString(mozilla::Omnijar::GRE, mGREURI);
     NS_ENSURE_SUCCESS(rv, rv);
 
-    //
-    // make resource://app/ point to the application directory or omnijar
-    //
-    rv = SetSubstitution(kAPP, uri);
-    NS_ENSURE_SUCCESS(rv, rv);
-
-    //
-    // make resource://gre/ point to the GRE directory
-    //
-    if (appURI.Length()) { // We already have greURI in uri if appURI.Length() is 0.
-        rv = NS_NewURI(getter_AddRefs(uri), greURI);
-        NS_ENSURE_SUCCESS(rv, rv);
+    // mozilla::Omnijar::GetURIString always returns a string ending with /,
+    // and we want to remove it.
+    mGREURI.Truncate(mGREURI.Length() - 1);
+    if (mAppURI.Length()) {
+      mAppURI.Truncate(mAppURI.Length() - 1);
+    } else {
+      mAppURI = mGREURI;
     }
 
-    rv = SetSubstitution(kGRE, uri);
-    NS_ENSURE_SUCCESS(rv, rv);
-
     //XXXbsmedberg Neil wants a resource://pchrome/ for the profile chrome dir...
     // but once I finish multiple chrome registration I'm not sure that it is needed
 
     // XXX dveditz: resource://pchrome/ defeats profile directory salting
     // if web content can load it. Tread carefully.
 
     return rv;
 }
@@ -95,8 +76,33 @@ nsResProtocolHandler::GetSubstitutionInt
         return NS_ERROR_NOT_AVAILABLE;
         
     rv = IOService()->NewFileURI(file, result);
     if (NS_FAILED(rv))
         return NS_ERROR_NOT_AVAILABLE;
 
     return NS_OK;
 }
+
+bool
+nsResProtocolHandler::ResolveSpecialCases(const nsACString& aHost,
+                                          const nsACString& aPath,
+                                          nsACString& aResult)
+{
+    if (aHost.Equals("") || aHost.Equals(kAPP)) {
+        aResult.Assign(mAppURI);
+    } else if (aHost.Equals(kGRE)) {
+        aResult.Assign(mGREURI);
+    } else {
+        return false;
+    }
+    aResult.Append(aPath);
+    return true;
+}
+
+nsresult
+nsResProtocolHandler::SetSubstitution(const nsACString& aRoot, nsIURI* aBaseURI)
+{
+    MOZ_ASSERT(!aRoot.Equals(""));
+    MOZ_ASSERT(!aRoot.Equals(kAPP));
+    MOZ_ASSERT(!aRoot.Equals(kGRE));
+    return SubstitutingProtocolHandler::SetSubstitution(aRoot, aBaseURI);
+}

--- a/netwerk/protocol/res/nsResProtocolHandler.h
+++ b/netwerk/protocol/res/nsResProtocolHandler.h
@@ -18,23 +18,46 @@ class nsResProtocolHandler final : publi
                                    public mozilla::SubstitutingProtocolHandler,
                                    public nsSupportsWeakReference
 {
 public:
     NS_DECL_ISUPPORTS_INHERITED
     NS_DECL_NSIRESPROTOCOLHANDLER
 
     NS_FORWARD_NSIPROTOCOLHANDLER(mozilla::SubstitutingProtocolHandler::)
-    NS_FORWARD_NSISUBSTITUTINGPROTOCOLHANDLER(mozilla::SubstitutingProtocolHandler::)
 
     nsResProtocolHandler()
       : SubstitutingProtocolHandler("resource", URI_STD | URI_IS_UI_RESOURCE | URI_IS_LOCAL_RESOURCE,
                                     /* aEnforceFileOrJar = */ false)
     {}
 
     nsresult Init();
 
+    NS_IMETHOD SetSubstitution(const nsACString& aRoot, nsIURI* aBaseURI) override;
+
+    NS_IMETHOD GetSubstitution(const nsACString& aRoot, nsIURI** aResult) override
+    {
+        return mozilla::SubstitutingProtocolHandler::GetSubstitution(aRoot, aResult);
+    }
+
+    NS_IMETHOD HasSubstitution(const nsACString& aRoot, bool* aResult) override
+    {
+        return mozilla::SubstitutingProtocolHandler::HasSubstitution(aRoot, aResult);
+    }
+
+    NS_IMETHOD ResolveURI(nsIURI *aResURI, nsACString& aResult) override
+    {
+        return mozilla::SubstitutingProtocolHandler::ResolveURI(aResURI, aResult);
+    }
+
 protected:
     nsresult GetSubstitutionInternal(const nsACString& aRoot, nsIURI** aResult) override;
     virtual ~nsResProtocolHandler() {}
+
+    bool ResolveSpecialCases(const nsACString& aHost, const nsACString& aPath,
+                             nsACString& aResult) override;
+
+private:
+    nsCString mAppURI;
+    nsCString mGREURI;
 };
 
 #endif /* nsResProtocolHandler_h___ */