diff --git a/caps/BasePrincipal.cpp b/caps/BasePrincipal.cpp
index 968f5e2b0f2f..90de4ec2ae18 100644
--- a/caps/BasePrincipal.cpp
+++ b/caps/BasePrincipal.cpp
@@ -324,6 +324,7 @@ OriginAttributes::SetFromGenericAttributes(const GenericOriginAttributes& aAttrs
   mFirstPartyDomain = aAttrs.mFirstPartyDomain;
 }
 
+/* static */
 bool
 OriginAttributes::IsFirstPartyEnabled()
 {
diff --git a/caps/BasePrincipal.h b/caps/BasePrincipal.h
index 689bce901f90..6a526e636873 100644
--- a/caps/BasePrincipal.h
+++ b/caps/BasePrincipal.h
@@ -61,13 +61,13 @@ public:
 
   void SetFromGenericAttributes(const GenericOriginAttributes& aAttrs);
 
+  // check if "privacy.firstparty.isolate" is enabled.
+  static bool IsFirstPartyEnabled();
+
 protected:
   OriginAttributes() {}
   explicit OriginAttributes(const OriginAttributesDictionary& aOther)
     : OriginAttributesDictionary(aOther) {}
-
-  // check if "privacy.firstparty.isolate" is enabled.
-  bool IsFirstPartyEnabled();
 };
 
 class PrincipalOriginAttributes;
diff --git a/docshell/base/nsDocShell.cpp b/docshell/base/nsDocShell.cpp
index 646378f0f366..5940d45aa44e 100644
--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -3573,8 +3573,54 @@ nsDocShell::CanAccessItem(nsIDocShellTreeItem* aTargetItem,
     return false;
   }
 
-  if (static_cast<nsDocShell*>(targetDS.get())->GetOriginAttributes() !=
-      static_cast<nsDocShell*>(accessingDS.get())->GetOriginAttributes()) {
+  nsCOMPtr<nsIDocShellTreeItem> accessingRoot;
+  aAccessingItem->GetSameTypeRootTreeItem(getter_AddRefs(accessingRoot));
+  nsCOMPtr<nsIDocShell> accessingRootDS = do_QueryInterface(accessingRoot);
+
+  nsCOMPtr<nsIDocShellTreeItem> targetRoot;
+  aTargetItem->GetSameTypeRootTreeItem(getter_AddRefs(targetRoot));
+  nsCOMPtr<nsIDocShell> targetRootDS = do_QueryInterface(targetRoot);
+
+  DocShellOriginAttributes targetOA =
+    static_cast<nsDocShell*>(targetDS.get())->GetOriginAttributes();
+  DocShellOriginAttributes accessingOA =
+    static_cast<nsDocShell*>(accessingDS.get())->GetOriginAttributes();
+
+  // When the first party isolation is on, the top-level docShell may not have
+  // the firstPartyDomain in its originAttributes, but its document will have
+  // it. So we get the firstPartyDomain from the nodePrincipal of the document
+  // before we compare the originAttributes.
+  if (OriginAttributes::IsFirstPartyEnabled()) {
+    if (accessingDS == accessingRootDS &&
+        aAccessingItem->ItemType() == nsIDocShellTreeItem::typeContent &&
+        !accessingDS->GetIsMozBrowserOrApp()) {
+
+      nsCOMPtr<nsIDocument> accessingDoc = aAccessingItem->GetDocument();
+
+      if (accessingDoc) {
+        nsCOMPtr<nsIPrincipal> accessingPrincipal = accessingDoc->NodePrincipal();
+
+        accessingOA.mFirstPartyDomain =
+          BasePrincipal::Cast(accessingPrincipal)->OriginAttributesRef().mFirstPartyDomain;
+      }
+    }
+
+    if (targetDS == targetRootDS &&
+        aTargetItem->ItemType() == nsIDocShellTreeItem::typeContent &&
+        !targetDS->GetIsMozBrowserOrApp()) {
+
+      nsCOMPtr<nsIDocument> targetDoc = aAccessingItem->GetDocument();
+
+      if (targetDoc) {
+        nsCOMPtr<nsIPrincipal> targetPrincipal = targetDoc->NodePrincipal();
+
+        targetOA.mFirstPartyDomain =
+          BasePrincipal::Cast(targetPrincipal)->OriginAttributesRef().mFirstPartyDomain;
+      }
+    }
+  }
+
+  if (targetOA != accessingOA) {
     return false;
   }
 
@@ -3584,10 +3630,6 @@ nsDocShell::CanAccessItem(nsIDocShellTreeItem* aTargetItem,
     return false;
   }
 
-
-  nsCOMPtr<nsIDocShellTreeItem> accessingRoot;
-  aAccessingItem->GetSameTypeRootTreeItem(getter_AddRefs(accessingRoot));
-
   if (aTargetItem == accessingRoot) {
     // A frame can navigate its root.
     return true;
@@ -3605,9 +3647,6 @@ nsDocShell::CanAccessItem(nsIDocShellTreeItem* aTargetItem,
     parent.swap(target);
   } while (target);
 
-  nsCOMPtr<nsIDocShellTreeItem> targetRoot;
-  aTargetItem->GetSameTypeRootTreeItem(getter_AddRefs(targetRoot));
-
   if (aTargetItem != targetRoot) {
     // target is a subframe, not in accessor's frame hierarchy, and all its
     // ancestors have origins different from that of the accessor. Don't