diff --git a/caps/OriginAttributes.cpp b/caps/OriginAttributes.cpp
index 4d7ddf1b1480..bc4abe31a965 100644
--- a/caps/OriginAttributes.cpp
+++ b/caps/OriginAttributes.cpp
@@ -82,6 +82,8 @@ void OriginAttributes::SetFirstPartyDomain(const bool aIsTopLevelDocument,
     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);
@@ -97,6 +99,15 @@ void OriginAttributes::SetFirstPartyDomain(const bool aIsTopLevelDocument,
     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,
diff --git a/caps/tests/gtest/TestOriginAttributes.cpp b/caps/tests/gtest/TestOriginAttributes.cpp
index 91416c7201cf..1c3428c4f398 100644
--- a/caps/tests/gtest/TestOriginAttributes.cpp
+++ b/caps/tests/gtest/TestOriginAttributes.cpp
@@ -3,8 +3,11 @@
  * 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;
@@ -17,6 +20,14 @@ static void TestSuffix(const OriginAttributes& attrs) {
   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;
@@ -34,3 +45,21 @@ 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);
+}