diff --git a/ipc/glue/BackgroundUtils.cpp b/ipc/glue/BackgroundUtils.cpp
index fa141d2ff7d2..f9f5ede6d556 100644
--- a/ipc/glue/BackgroundUtils.cpp
+++ b/ipc/glue/BackgroundUtils.cpp
@@ -387,6 +387,7 @@ LoadInfoToLoadInfoArgs(nsILoadInfo *aLoadInfo,
       aLoadInfo->GetInnerWindowID(),
       aLoadInfo->GetOuterWindowID(),
       aLoadInfo->GetParentOuterWindowID(),
+      aLoadInfo->GetTopOuterWindowID(),
       aLoadInfo->GetFrameOuterWindowID(),
       aLoadInfo->GetEnforceSecurity(),
       aLoadInfo->GetInitialSecurityCheckDone(),
@@ -482,6 +483,7 @@ LoadInfoArgsToLoadInfo(const OptionalLoadInfoArgs& aOptionalLoadInfoArgs,
                           loadInfoArgs.innerWindowID(),
                           loadInfoArgs.outerWindowID(),
                           loadInfoArgs.parentOuterWindowID(),
+                          loadInfoArgs.topOuterWindowID(),
                           loadInfoArgs.frameOuterWindowID(),
                           loadInfoArgs.enforceSecurity(),
                           loadInfoArgs.initialSecurityCheckDone(),
diff --git a/netwerk/base/LoadInfo.cpp b/netwerk/base/LoadInfo.cpp
index e319cc876a5d..594df3474bdb 100644
--- a/netwerk/base/LoadInfo.cpp
+++ b/netwerk/base/LoadInfo.cpp
@@ -29,6 +29,16 @@ using namespace mozilla::dom;
 namespace mozilla {
 namespace net {
 
+static uint64_t
+FindTopOuterWindowID(nsPIDOMWindowOuter* aOuter)
+{
+  nsCOMPtr<nsPIDOMWindowOuter> outer = aOuter;
+  while (nsCOMPtr<nsPIDOMWindowOuter> parent = outer->GetScriptableParentOrNull()) {
+    outer = parent;
+  }
+  return outer->WindowID();
+}
+
 LoadInfo::LoadInfo(nsIPrincipal* aLoadingPrincipal,
                    nsIPrincipal* aTriggeringPrincipal,
                    nsINode* aLoadingContext,
@@ -51,6 +61,7 @@ LoadInfo::LoadInfo(nsIPrincipal* aLoadingPrincipal,
   , mInnerWindowID(0)
   , mOuterWindowID(0)
   , mParentOuterWindowID(0)
+  , mTopOuterWindowID(0)
   , mFrameOuterWindowID(0)
   , mEnforceSecurity(false)
   , mInitialSecurityCheckDone(false)
@@ -103,6 +114,7 @@ LoadInfo::LoadInfo(nsIPrincipal* aLoadingPrincipal,
       mOuterWindowID = contextOuter->WindowID();
       nsCOMPtr<nsPIDOMWindowOuter> parent = contextOuter->GetScriptableParent();
       mParentOuterWindowID = parent ? parent->WindowID() : mOuterWindowID;
+      mTopOuterWindowID = FindTopOuterWindowID(contextOuter);
     }
 
     mInnerWindowID = aLoadingContext->OwnerDoc()->InnerWindowID();
@@ -228,6 +240,7 @@ LoadInfo::LoadInfo(nsPIDOMWindowOuter* aOuterWindow,
   , mInnerWindowID(0)
   , mOuterWindowID(0)
   , mParentOuterWindowID(0)
+  , mTopOuterWindowID(0)
   , mFrameOuterWindowID(0)
   , mEnforceSecurity(false)
   , mInitialSecurityCheckDone(false)
@@ -258,6 +271,7 @@ LoadInfo::LoadInfo(nsPIDOMWindowOuter* aOuterWindow,
   // with the hidden window.
   nsCOMPtr<nsPIDOMWindowOuter> parent = aOuterWindow->GetScriptableParent();
   mParentOuterWindowID = parent ? parent->WindowID() : 0;
+  mTopOuterWindowID = FindTopOuterWindowID(aOuterWindow);
 
   // get the docshell from the outerwindow, and then get the originattributes
   nsCOMPtr<nsIDocShell> docShell = aOuterWindow->GetDocShell();
@@ -290,6 +304,7 @@ LoadInfo::LoadInfo(const LoadInfo& rhs)
   , mInnerWindowID(rhs.mInnerWindowID)
   , mOuterWindowID(rhs.mOuterWindowID)
   , mParentOuterWindowID(rhs.mParentOuterWindowID)
+  , mTopOuterWindowID(rhs.mTopOuterWindowID)
   , mFrameOuterWindowID(rhs.mFrameOuterWindowID)
   , mEnforceSecurity(rhs.mEnforceSecurity)
   , mInitialSecurityCheckDone(rhs.mInitialSecurityCheckDone)
@@ -323,6 +338,7 @@ LoadInfo::LoadInfo(nsIPrincipal* aLoadingPrincipal,
                    uint64_t aInnerWindowID,
                    uint64_t aOuterWindowID,
                    uint64_t aParentOuterWindowID,
+                   uint64_t aTopOuterWindowID,
                    uint64_t aFrameOuterWindowID,
                    bool aEnforceSecurity,
                    bool aInitialSecurityCheckDone,
@@ -351,6 +367,7 @@ LoadInfo::LoadInfo(nsIPrincipal* aLoadingPrincipal,
   , mInnerWindowID(aInnerWindowID)
   , mOuterWindowID(aOuterWindowID)
   , mParentOuterWindowID(aParentOuterWindowID)
+  , mTopOuterWindowID(aTopOuterWindowID)
   , mFrameOuterWindowID(aFrameOuterWindowID)
   , mEnforceSecurity(aEnforceSecurity)
   , mInitialSecurityCheckDone(aInitialSecurityCheckDone)
@@ -724,6 +741,13 @@ LoadInfo::GetParentOuterWindowID(uint64_t* aResult)
   return NS_OK;
 }
 
+NS_IMETHODIMP
+LoadInfo::GetTopOuterWindowID(uint64_t* aResult)
+{
+  *aResult = mTopOuterWindowID;
+  return NS_OK;
+}
+
 NS_IMETHODIMP
 LoadInfo::GetFrameOuterWindowID(uint64_t* aResult)
 {
diff --git a/netwerk/base/LoadInfo.h b/netwerk/base/LoadInfo.h
index 7e670bb0f9b0..2f97d3276fe3 100644
--- a/netwerk/base/LoadInfo.h
+++ b/netwerk/base/LoadInfo.h
@@ -107,6 +107,7 @@ private:
            uint64_t aInnerWindowID,
            uint64_t aOuterWindowID,
            uint64_t aParentOuterWindowID,
+           uint64_t aTopOuterWindowID,
            uint64_t aFrameOuterWindowID,
            bool aEnforceSecurity,
            bool aInitialSecurityCheckDone,
@@ -159,6 +160,7 @@ private:
   uint64_t                         mInnerWindowID;
   uint64_t                         mOuterWindowID;
   uint64_t                         mParentOuterWindowID;
+  uint64_t                         mTopOuterWindowID;
   uint64_t                         mFrameOuterWindowID;
   bool                             mEnforceSecurity;
   bool                             mInitialSecurityCheckDone;
diff --git a/netwerk/base/nsILoadInfo.idl b/netwerk/base/nsILoadInfo.idl
index c0ef95bd4ece..1fcc33e5e579 100644
--- a/netwerk/base/nsILoadInfo.idl
+++ b/netwerk/base/nsILoadInfo.idl
@@ -507,15 +507,17 @@ interface nsILoadInfo : nsISupports
   /**
    * These are the window IDs of the window in which the element being
    * loaded lives. parentOuterWindowID is the window ID of this window's
-   * parent.
+   * parent. topOuterWindowID is the ID of the top-level window of the same
+   * docShell type.
    *
    * Note that these window IDs can be 0 if the window is not
-   * available. parentOuterWindowID will be the same as outerWindowID if the
-   * window has no parent.
+   * available. parentOuterWindowID and topOuterWindowID will be the same as
+   * outerWindowID if the window has no parent.
    */
   [infallible] readonly attribute unsigned long long innerWindowID;
   [infallible] readonly attribute unsigned long long outerWindowID;
   [infallible] readonly attribute unsigned long long parentOuterWindowID;
+  [infallible] readonly attribute unsigned long long topOuterWindowID;
 
   /**
    * Only when the element being loaded is <frame src="foo.html">
diff --git a/netwerk/ipc/NeckoChannelParams.ipdlh b/netwerk/ipc/NeckoChannelParams.ipdlh
index 2426f72205a9..4c3d40c3f8fd 100644
--- a/netwerk/ipc/NeckoChannelParams.ipdlh
+++ b/netwerk/ipc/NeckoChannelParams.ipdlh
@@ -51,6 +51,7 @@ struct LoadInfoArgs
   uint64_t                    innerWindowID;
   uint64_t                    outerWindowID;
   uint64_t                    parentOuterWindowID;
+  uint64_t                    topOuterWindowID;
   uint64_t                    frameOuterWindowID;
   bool                        enforceSecurity;
   bool                        initialSecurityCheckDone;