diff --git a/docshell/base/nsDocShell.cpp b/docshell/base/nsDocShell.cpp
index eac9c1e424a1..636210ccc5b3 100644
--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -13464,35 +13464,6 @@ nsCommandManager* nsDocShell::GetCommandManager() {
   return mCommandManager;
 }
 
-NS_IMETHODIMP
-nsDocShell::GetIsOnlyToplevelInTabGroup(bool* aResult) {
-  MOZ_ASSERT(aResult);
-
-  nsPIDOMWindowOuter* outer = GetWindow();
-  MOZ_ASSERT(outer);
-
-  // If we are not toplevel then we are not the only toplevel window in the
-  // tab group.
-  if (outer->GetInProcessScriptableParentOrNull()) {
-    *aResult = false;
-    return NS_OK;
-  }
-
-  // If we have any other toplevel windows in our tab group, then we are not
-  // the only toplevel window in the tab group.
-  nsTArray<nsPIDOMWindowOuter*> toplevelWindows =
-      outer->TabGroup()->GetTopLevelWindows();
-  if (toplevelWindows.Length() > 1) {
-    *aResult = false;
-    return NS_OK;
-  }
-  MOZ_ASSERT(toplevelWindows.Length() == 1);
-  MOZ_ASSERT(toplevelWindows[0] == outer);
-
-  *aResult = true;
-  return NS_OK;
-}
-
 NS_IMETHODIMP
 nsDocShell::GetAwaitingLargeAlloc(bool* aResult) {
   MOZ_ASSERT(aResult);
diff --git a/docshell/base/nsIDocShell.idl b/docshell/base/nsIDocShell.idl
index 7dd8692e3427..2841d3a1abd8 100644
--- a/docshell/base/nsIDocShell.idl
+++ b/docshell/base/nsIDocShell.idl
@@ -1055,20 +1055,6 @@ interface nsIDocShell : nsIDocShellTreeItem
    */
   [infallible] attribute nsIDocShell_MetaViewportOverride metaViewportOverride;
 
-  /**
-   * This value is `true` if its corresponding unit of related browsing contexts
-   * (TabGroup) contains only 1 toplevel window, and that window is the outer
-   * window corresponding to this docshell.
-   *
-   * The value is `false` otherwise. This is the case if the docshell is an
-   * iframe, has window.opener set, or another window with window.opener
-   * referring to this window exists.
-   *
-   * If this value is `false`, it would be web content visible for a load
-   * occuring in this docshell to be performed within a different docshell.
-   */
-  [infallible] readonly attribute boolean isOnlyToplevelInTabGroup;
-
   /**
    * Returns `true` if this docshell was created due to a Large-Allocation
    * header, and has not seen the initiating load yet.
diff --git a/dom/base/TabGroup.cpp b/dom/base/TabGroup.cpp
index 2efcbb041408..7af4257d3806 100644
--- a/dom/base/TabGroup.cpp
+++ b/dom/base/TabGroup.cpp
@@ -204,20 +204,6 @@ void TabGroup::MaybeDestroy() {
   }
 }
 
-nsTArray<nsPIDOMWindowOuter*> TabGroup::GetTopLevelWindows() const {
-  MOZ_ASSERT(NS_IsMainThread());
-  nsTArray<nsPIDOMWindowOuter*> array;
-
-  for (nsPIDOMWindowOuter* outerWindow : mWindows) {
-    if (outerWindow->GetDocShell() &&
-        !outerWindow->GetInProcessScriptableParentOrNull()) {
-      array.AppendElement(outerWindow);
-    }
-  }
-
-  return array;
-}
-
 TabGroup::HashEntry::HashEntry(const nsACString* aKey)
     : nsCStringHashKey(aKey), mDocGroup(nullptr) {}
 
diff --git a/dom/base/TabGroup.h b/dom/base/TabGroup.h
index a8b080ad6972..0e9bc9520028 100644
--- a/dom/base/TabGroup.h
+++ b/dom/base/TabGroup.h
@@ -101,7 +101,6 @@ class TabGroup final : public SchedulerGroup,
   // Count with 'aActiveOnly' = true
   uint32_t Count(bool aActiveOnly = false) const;
 
-  nsTArray<nsPIDOMWindowOuter*> GetTopLevelWindows() const;
   const nsTArray<nsPIDOMWindowOuter*>& GetWindows() { return mWindows; }
 
   // This method is always safe to call off the main thread. The nsIEventTarget
diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index 6300598c88aa..a3cd66e2dcd1 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -45,6 +45,8 @@
 #include "mozilla/DebugOnly.h"
 #include "mozilla/LoadInfo.h"
 #include "mozilla/dom/BlobURLProtocolHandler.h"
+#include "mozilla/dom/BrowsingContext.h"
+#include "mozilla/dom/BrowsingContextGroup.h"
 #include "mozilla/dom/ContentParent.h"
 #include "mozilla/dom/ContentChild.h"
 #include "mozilla/dom/CustomElementRegistry.h"
@@ -9243,7 +9245,11 @@ bool nsContentUtils::AttemptLargeAllocationLoad(nsIHttpChannel* aChannel) {
   }
 
   nsIDocShell* docShell = outer->GetDocShell();
-  if (!docShell->GetIsOnlyToplevelInTabGroup()) {
+  BrowsingContext* browsingContext = docShell->GetBrowsingContext();
+  bool isOnlyToplevelBrowsingContext =
+      !browsingContext->GetParent() &&
+      browsingContext->Group()->Toplevels().Length() == 1;
+  if (!isOnlyToplevelBrowsingContext) {
     outer->SetLargeAllocStatus(LargeAllocStatus::NOT_ONLY_TOPLEVEL_IN_TABGROUP);
     return false;
   }
diff --git a/toolkit/modules/E10SUtils.jsm b/toolkit/modules/E10SUtils.jsm
index 7803a4e748c5..af15de4f7202 100644
--- a/toolkit/modules/E10SUtils.jsm
+++ b/toolkit/modules/E10SUtils.jsm
@@ -659,11 +659,14 @@ var E10SUtils = {
     // to change processes, we want to load into a new process so that we can throw
     // this one out. We don't want to move into a new process if we have post data,
     // because we would accidentally throw out that data.
+    let isOnlyToplevelBrowsingContext =
+      !aDocShell.browsingContext.parent &&
+      aDocShell.browsingContext.group.getToplevels().length == 1;
     if (
       !aHasPostData &&
       Services.appinfo.remoteType == LARGE_ALLOCATION_REMOTE_TYPE &&
       !aDocShell.awaitingLargeAlloc &&
-      aDocShell.isOnlyToplevelInTabGroup
+      isOnlyToplevelBrowsingContext
     ) {
       return false;
     }