--- a/caps/OriginAttributes.cpp
+++ b/caps/OriginAttributes.cpp
@@ -77,31 +77,42 @@ void OriginAttributes::SetFirstPartyDoma
       mFirstPartyDomain.AppendLiteral("]");
     } else {
       mFirstPartyDomain = NS_ConvertUTF8toUTF16(ipAddr);
     }
 
     return;
   }
 
+  // Saving isInsufficientDomainLevels before rv is overwritten.
+  bool isInsufficientDomainLevels = (rv == NS_ERROR_INSUFFICIENT_DOMAIN_LEVELS);
   nsAutoCString scheme;
   rv = aURI->GetScheme(scheme);
   NS_ENSURE_SUCCESS_VOID(rv);
   if (scheme.EqualsLiteral("about")) {
     mFirstPartyDomain.AssignLiteral(ABOUT_URI_FIRST_PARTY_DOMAIN);
     return;
   }
 
   nsCOMPtr<nsIPrincipal> blobPrincipal;
   if (dom::BlobURLProtocolHandler::GetBlobURLPrincipal(
           aURI, getter_AddRefs(blobPrincipal))) {
     MOZ_ASSERT(blobPrincipal);
     mFirstPartyDomain = blobPrincipal->OriginAttributesRef().mFirstPartyDomain;
     return;
   }
+
+  if (isInsufficientDomainLevels) {
+    nsAutoCString publicSuffix;
+    rv = tldService->GetPublicSuffix(aURI, publicSuffix);
+    if (NS_SUCCEEDED(rv)) {
+      mFirstPartyDomain = NS_ConvertUTF8toUTF16(publicSuffix);
+    }
+    return;
+  }
 }
 
 void OriginAttributes::SetFirstPartyDomain(const bool aIsTopLevelDocument,
                                            const nsACString& aDomain) {
   bool isFirstPartyEnabled = IsFirstPartyEnabled();
 
   // If the pref is off or this is not a top level load, bail out.
   if (!isFirstPartyEnabled || !aIsTopLevelDocument) {

--- a/caps/tests/gtest/TestOriginAttributes.cpp
+++ b/caps/tests/gtest/TestOriginAttributes.cpp
@@ -1,27 +1,38 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * 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 "gtest/gtest.h"
 #include "mozilla/BasePrincipal.h"
+#include "mozilla/Preferences.h"
+#include "nsNetUtil.h"
 
 using mozilla::OriginAttributes;
+using mozilla::Preferences;
 
 static void TestSuffix(const OriginAttributes& attrs) {
   nsAutoCString suffix;
   attrs.CreateSuffix(suffix);
 
   OriginAttributes attrsFromSuffix;
   bool success = attrsFromSuffix.PopulateFromSuffix(suffix);
   EXPECT_TRUE(success);
 
   EXPECT_EQ(attrs, attrsFromSuffix);
 }
 
+static void TestFPD(const nsAString &spec, const nsAString &fpd) {
+  OriginAttributes attrs;
+  nsCOMPtr<nsIURI> url;
+  ASSERT_EQ(NS_NewURI(getter_AddRefs(url), spec), NS_OK);
+  attrs.SetFirstPartyDomain(true, url);
+  EXPECT_TRUE(attrs.mFirstPartyDomain.Equals(fpd));
+}
+
 TEST(OriginAttributes, Suffix_default)
 {
   OriginAttributes attrs;
   TestSuffix(attrs);
 }
 
 TEST(OriginAttributes, Suffix_appId_inIsolatedMozBrowser)
 {
@@ -29,8 +40,26 @@ TEST(OriginAttributes, Suffix_appId_inIs
   TestSuffix(attrs);
 }
 
 TEST(OriginAttributes, Suffix_maxAppId_inIsolatedMozBrowser)
 {
   OriginAttributes attrs(4294967295, true);
   TestSuffix(attrs);
 }
+
+TEST(OriginAttributes, FirstPartyDomain_default)
+{
+  static const char prefKey[] = "privacy.firstparty.isolate";
+  bool oldPref = Preferences::GetBool(prefKey);
+  Preferences::SetBool(prefKey, true);
+  TestFPD(NS_LITERAL_STRING("http://www.example.com"),
+          NS_LITERAL_STRING("example.com"));
+  TestFPD(NS_LITERAL_STRING("http://s3.amazonaws.com"),
+          NS_LITERAL_STRING("s3.amazonaws.com"));
+  TestFPD(NS_LITERAL_STRING("http://com"), NS_LITERAL_STRING("com"));
+  TestFPD(NS_LITERAL_STRING("http://.com"), NS_LITERAL_STRING(""));
+  TestFPD(NS_LITERAL_STRING("http://..com"), NS_LITERAL_STRING(""));
+  TestFPD(NS_LITERAL_STRING("http://127.0.0.1"),
+          NS_LITERAL_STRING("127.0.0.1"));
+  TestFPD(NS_LITERAL_STRING("http://[::1]"), NS_LITERAL_STRING("[::1]"));
+  Preferences::SetBool(prefKey, oldPref);
+}