diff --git a/browser/base/content/test/chrome/test_aboutCrashed.xhtml b/browser/base/content/test/chrome/test_aboutCrashed.xhtml
index 8bad1ac80d91..df48694e38aa 100644
--- a/browser/base/content/test/chrome/test_aboutCrashed.xhtml
+++ b/browser/base/content/test/chrome/test_aboutCrashed.xhtml
@@ -57,7 +57,7 @@
       frame1.docShell.chromeEventHandler.removeAttribute("crashedPageTitle");
 
       SimpleTest.is(frame1.contentDocument.documentURI,
-                    "about:tabcrashed?e=tabcrashed&u=http%3A//www.example.com/1&c=UTF-8&f=regular&d=pageTitle",
+                    "about:tabcrashed?e=tabcrashed&u=http%3A//www.example.com/1&c=UTF-8&d=pageTitle",
                     "Correct about:tabcrashed displayed for page with title.");
 
       errorPageReady = waitForErrorPage(frame2);
@@ -66,7 +66,7 @@
       await errorPageReady;
 
       SimpleTest.is(frame2.contentDocument.documentURI,
-                    "about:tabcrashed?e=tabcrashed&u=http%3A//www.example.com/2&c=UTF-8&f=regular&d=%20",
+                    "about:tabcrashed?e=tabcrashed&u=http%3A//www.example.com/2&c=UTF-8&d=%20",
                     "Correct about:tabcrashed displayed for page with no title.");
 
       SimpleTest.finish();
diff --git a/browser/base/content/test/chrome/test_aboutRestartRequired.xhtml b/browser/base/content/test/chrome/test_aboutRestartRequired.xhtml
index 17cd84bd73ac..4b8293e45001 100644
--- a/browser/base/content/test/chrome/test_aboutRestartRequired.xhtml
+++ b/browser/base/content/test/chrome/test_aboutRestartRequired.xhtml
@@ -55,7 +55,7 @@
       frame1.docShell.chromeEventHandler.removeAttribute("crashedPageTitle");
 
       SimpleTest.is(frame1.contentDocument.documentURI,
-                    "about:restartrequired?e=restartrequired&u=http%3A//www.example.com/1&c=UTF-8&f=regular&d=%20",
+                    "about:restartrequired?e=restartrequired&u=http%3A//www.example.com/1&c=UTF-8&d=%20",
                     "Correct about:restartrequired displayed for page with title.");
 
       errorPageReady = waitForErrorPage(frame2);
@@ -64,7 +64,7 @@
       await errorPageReady;
 
       SimpleTest.is(frame2.contentDocument.documentURI,
-                    "about:restartrequired?e=restartrequired&u=http%3A//www.example.com/2&c=UTF-8&f=regular&d=%20",
+                    "about:restartrequired?e=restartrequired&u=http%3A//www.example.com/2&c=UTF-8&d=%20",
                     "Correct about:restartrequired displayed for page with no title.");
 
       SimpleTest.finish();
diff --git a/docshell/base/nsDocShell.cpp b/docshell/base/nsDocShell.cpp
index fda296234dc0..ebd58b60a126 100644
--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -354,7 +354,6 @@ nsDocShell::nsDocShell(BrowsingContext* aBrowsingContext,
       mLoadType(0),
       mDefaultLoadFlags(nsIRequest::LOAD_NORMAL),
       mFailedLoadType(0),
-      mFrameType(FRAME_TYPE_REGULAR),
       mDisplayMode(nsIDocShell::DISPLAY_MODE_BROWSER),
       mJSRunToCompletionDepth(0),
       mTouchEventsOverride(nsIDocShell::TOUCHEVENTS_OVERRIDE_NONE),
@@ -1847,21 +1846,6 @@ nsDocShell::GetFullscreenAllowed(bool* aFullscreenAllowed) {
   return parent->GetFullscreenAllowed(aFullscreenAllowed);
 }
 
-NS_IMETHODIMP
-nsDocShell::SetFullscreenAllowed(bool aFullscreenAllowed) {
-  if (!nsIDocShell::GetIsMozBrowser()) {
-    // Only allow setting of fullscreenAllowed on content/process boundaries.
-    // At non-boundaries the fullscreenAllowed attribute is calculated based on
-    // whether all enclosing frames have the "mozFullscreenAllowed" attribute
-    // set to "true". fullscreenAllowed is set at the process boundaries to
-    // propagate the value of the parent's "mozFullscreenAllowed" attribute
-    // across process boundaries.
-    return NS_ERROR_UNEXPECTED;
-  }
-  mFullscreenAllowed = (aFullscreenAllowed ? PARENT_ALLOWS : PARENT_PROHIBITS);
-  return NS_OK;
-}
-
 hal::ScreenOrientation nsDocShell::OrientationLock() {
   return mOrientationLock;
 }
@@ -2638,21 +2622,8 @@ void nsDocShell::MaybeClearStorageAccessFlag() {
 
 NS_IMETHODIMP
 nsDocShell::GetInProcessSameTypeParent(nsIDocShellTreeItem** aParent) {
-  NS_ENSURE_ARG_POINTER(aParent);
-  *aParent = nullptr;
-
-  if (nsIDocShell::GetIsMozBrowser()) {
-    return NS_OK;
-  }
-
-  nsCOMPtr<nsIDocShellTreeItem> parent =
-      do_QueryInterface(GetAsSupports(mParent));
-  if (!parent) {
-    return NS_OK;
-  }
-
-  if (parent->ItemType() == mItemType) {
-    parent.swap(*aParent);
+  if (BrowsingContext* parentBC = mBrowsingContext->GetParent()) {
+    *aParent = do_AddRef(parentBC->GetDocShell()).take();
   }
   return NS_OK;
 }
@@ -3896,10 +3867,6 @@ nsresult nsDocShell::LoadErrorPage(nsIURI* aURI, const char16_t* aURL,
   }
   errorPageUrl.AppendLiteral("&c=UTF-8");
 
-  nsAutoCString frameType(FrameTypeToString(mFrameType));
-  errorPageUrl.AppendLiteral("&f=");
-  errorPageUrl.AppendASCII(frameType.get());
-
   nsCOMPtr<nsICaptivePortalService> cps = do_GetService(NS_CAPTIVEPORTAL_CID);
   int32_t cpsState;
   if (cps && NS_SUCCEEDED(cps->GetState(&cpsState)) &&
@@ -4723,9 +4690,7 @@ nsDocShell::SetIsActive(bool aIsActive) {
       continue;
     }
 
-    if (!docshell->GetIsMozBrowser()) {
-      docshell->SetIsActive(aIsActive);
-    }
+    docshell->SetIsActive(aIsActive);
   }
 
   // Restart or stop meta refresh timers if necessary
@@ -9014,8 +8979,7 @@ nsresult nsDocShell::InternalLoad(nsDocShellLoadState* aLoadState,
     }
   }
 
-  bool isTopLevelDoc =
-      mItemType == typeContent && (!IsFrame() || GetIsMozBrowser());
+  bool isTopLevelDoc = mBrowsingContext->IsTopContent();
 
   OriginAttributes attrs = GetOriginAttributes();
   attrs.SetFirstPartyDomain(isTopLevelDoc, aLoadState->URI());
@@ -12395,46 +12359,6 @@ nsDocShell::GetCanExecuteScripts(bool* aResult) {
   return NS_OK;
 }
 
-/* [infallible] */
-NS_IMETHODIMP nsDocShell::SetFrameType(FrameType aFrameType) {
-  mFrameType = aFrameType;
-  return NS_OK;
-}
-
-/* [infallible] */
-NS_IMETHODIMP nsDocShell::GetFrameType(FrameType* aFrameType) {
-  *aFrameType = mFrameType;
-  return NS_OK;
-}
-
-/* [infallible] */
-NS_IMETHODIMP nsDocShell::GetIsMozBrowser(bool* aIsMozBrowser) {
-  *aIsMozBrowser = (mFrameType == FRAME_TYPE_BROWSER);
-  return NS_OK;
-}
-
-uint32_t nsDocShell::GetInheritedFrameType() {
-  if (mFrameType != FRAME_TYPE_REGULAR) {
-    return mFrameType;
-  }
-
-  nsCOMPtr<nsIDocShellTreeItem> parentAsItem;
-  GetInProcessSameTypeParent(getter_AddRefs(parentAsItem));
-
-  nsCOMPtr<nsIDocShell> parent = do_QueryInterface(parentAsItem);
-  if (!parent) {
-    return FRAME_TYPE_REGULAR;
-  }
-
-  return static_cast<nsDocShell*>(parent.get())->GetInheritedFrameType();
-}
-
-/* [infallible] */
-NS_IMETHODIMP nsDocShell::GetIsInMozBrowser(bool* aIsInMozBrowser) {
-  *aIsInMozBrowser = (GetInheritedFrameType() == FRAME_TYPE_BROWSER);
-  return NS_OK;
-}
-
 /* [infallible] */
 NS_IMETHODIMP nsDocShell::GetIsTopLevelContentDocShell(
     bool* aIsTopLevelContentDocShell) {
diff --git a/docshell/base/nsDocShell.h b/docshell/base/nsDocShell.h
index c07b8e6fd151..f7f8916af6b1 100644
--- a/docshell/base/nsDocShell.h
+++ b/docshell/base/nsDocShell.h
@@ -569,18 +569,6 @@ class nsDocShell final : public nsDocLoader,
     return uint32_t(aTimeUsec / PR_USEC_PER_SEC);
   }
 
-  static const nsCString FrameTypeToString(uint32_t aFrameType) {
-    switch (aFrameType) {
-      case FRAME_TYPE_BROWSER:
-        return NS_LITERAL_CSTRING("browser");
-      case FRAME_TYPE_REGULAR:
-        return NS_LITERAL_CSTRING("regular");
-      default:
-        NS_ERROR("Unknown frame type");
-        return EmptyCString();
-    }
-  }
-
   virtual ~nsDocShell();
 
   //
@@ -1009,7 +997,6 @@ class nsDocShell final : public nsDocLoader,
   bool IsPrintingOrPP(bool aDisplayErrorDialog = true);
   bool IsNavigationAllowed(bool aDisplayPrintErrorDialog = true,
                            bool aCheckIfUnloadFired = true);
-  uint32_t GetInheritedFrameType();
   nsIScrollableFrame* GetRootScrollFrame();
   nsIChannel* GetCurrentDocChannel();
   nsresult EnsureScriptEnvironment();
@@ -1223,9 +1210,6 @@ class nsDocShell final : public nsDocLoader,
   uint32_t mDefaultLoadFlags;
   uint32_t mFailedLoadType;
 
-  // Are we a regular frame, a browser frame, or an app frame?
-  FrameType mFrameType;
-
   // This represents the CSS display-mode we are currently using. This is mostly
   // used for media queries.
   DisplayMode mDisplayMode;
diff --git a/docshell/base/nsIDocShell.idl b/docshell/base/nsIDocShell.idl
index f730653b9614..700845dbd143 100644
--- a/docshell/base/nsIDocShell.idl
+++ b/docshell/base/nsIDocShell.idl
@@ -684,32 +684,6 @@ interface nsIDocShell : nsIDocShellTreeItem
    */
   [noscript] void notifyScrollObservers();
 
-  /**
-   * The type of iframe that this docshell lives.
-   */
-  cenum FrameType : 8 {
-    FRAME_TYPE_REGULAR = 0,
-    FRAME_TYPE_BROWSER = 1,
-  };
-  [infallible] attribute nsIDocShell_FrameType frameType;
-
-  /**
-   * Returns true if this docshell corresponds to an <iframe mozbrowser>.
-   * <xul:browser> returns false here.
-   */
-  [infallible] readonly attribute boolean isMozBrowser;
-
-  /**
-   * Returns true if this docshell corresponds to an <iframe mozbrowser>, or
-   * if this docshell is contained in an <iframe mozbrowser>. <xul:browser>
-   * returns false here.
-   *
-   * To compute this value, we walk up the docshell hierarchy.  If we encounter
-   * a docshell with isMozBrowser before we hit the end of the hierarchy,
-   * we return true.  Otherwise, we return false.
-   */
-  [infallible] readonly attribute boolean isInMozBrowser;
-
   /**
    * Returns true if this docshell is the top level content docshell.
    */
@@ -717,7 +691,7 @@ interface nsIDocShell : nsIDocShellTreeItem
 
   /**
    * Like nsIDocShellTreeItem::GetSameTypeParent, except this ignores <iframe
-   * mozbrowser> boundaries.
+   * mozbrowser> boundaries. Which no longer exist.
    *
    * @deprecated: Use `BrowsingContext::GetParent()` in the future.
    */
@@ -779,8 +753,6 @@ interface nsIDocShell : nsIDocShellTreeItem
    */
   [infallible] readonly attribute boolean fullscreenAllowed;
 
-  void setFullscreenAllowed(in boolean allowed);
-
   [notxpcom] uint32_t orientationLock();
   [notxpcom] void setOrientationLock(in uint32_t orientationLock);
 
diff --git a/dom/base/nsFrameLoader.cpp b/dom/base/nsFrameLoader.cpp
index d4f9db548ba2..12a5ae8fcbb0 100644
--- a/dom/base/nsFrameLoader.cpp
+++ b/dom/base/nsFrameLoader.cpp
@@ -2144,13 +2144,9 @@ nsresult nsFrameLoader::MaybeCreateDocShell() {
     NS_ENSURE_SUCCESS(rv, rv);
   }
 
-  if (OwnerIsMozBrowserFrame()) {
-    docShell->SetFrameType(nsIDocShell::FRAME_TYPE_BROWSER);
-  }
-
   // Apply sandbox flags even if our owner is not an iframe, as this copies
   // flags from our owning content's owning document.
-  // Note: ApplySandboxFlags should be called after docShell->SetFrameType
+  // Note: ApplySandboxFlags should be called after docShell->SetIsFrame
   // because we need to get the correct presentation URL in ApplySandboxFlags.
   uint32_t sandboxFlags = 0;
   HTMLIFrameElement* iframe = HTMLIFrameElement::FromNode(mOwnerContent);
@@ -2165,10 +2161,6 @@ nsresult nsFrameLoader::MaybeCreateDocShell() {
     if (mOwnerContent->GetAttr(kNameSpaceID_None, nsGkAtoms::name, name)) {
       docShell->SetName(name);
     }
-    docShell->SetFullscreenAllowed(
-        mOwnerContent->HasAttr(kNameSpaceID_None, nsGkAtoms::allowfullscreen) ||
-        mOwnerContent->HasAttr(kNameSpaceID_None,
-                               nsGkAtoms::mozallowfullscreen));
   }
 
   // Typically there will be a window, however for some cases such as printing
@@ -2176,8 +2168,7 @@ nsresult nsFrameLoader::MaybeCreateDocShell() {
   // that the window exists to ensure we don't try to gather ancestors for
   // those cases.
   nsCOMPtr<nsPIDOMWindowOuter> win = doc->GetWindow();
-  if (!docShell->GetIsMozBrowser() &&
-      parentDocShell->ItemType() == docShell->ItemType() &&
+  if (parentDocShell->ItemType() == docShell->ItemType() &&
       !doc->IsStaticDocument() && win) {
     // Propagate through the ancestor principals.
     nsTArray<nsCOMPtr<nsIPrincipal>> ancestorPrincipals;
diff --git a/dom/base/nsGlobalWindowOuter.cpp b/dom/base/nsGlobalWindowOuter.cpp
index cfe5811fe11f..ff5e2294e23e 100644
--- a/dom/base/nsGlobalWindowOuter.cpp
+++ b/dom/base/nsGlobalWindowOuter.cpp
@@ -3184,12 +3184,12 @@ nsPIDOMWindowOuter* nsGlobalWindowOuter::GetInProcessScriptableParent() {
     return nullptr;
   }
 
-  if (mDocShell->GetIsMozBrowser()) {
-    return this;
+  if (BrowsingContext* parentBC = GetBrowsingContext()->GetParent()) {
+    if (nsCOMPtr<nsPIDOMWindowOuter> parent = parentBC->GetDOMWindow()) {
+      return parent;
+    }
   }
-
-  nsCOMPtr<nsPIDOMWindowOuter> parent = GetInProcessParent();
-  return parent;
+  return this;
 }
 
 /**
@@ -3362,13 +3362,6 @@ already_AddRefed<nsPIDOMWindowOuter> nsGlobalWindowOuter::GetContentInternal(
     return content.forget();
   }
 
-  // If we're contained in <iframe mozbrowser>, then GetContent is the same as
-  // window.top.
-  if (mDocShell && mDocShell->GetIsInMozBrowser()) {
-    nsCOMPtr<nsPIDOMWindowOuter> domWindow(GetInProcessScriptableTop());
-    return domWindow.forget();
-  }
-
   nsCOMPtr<nsIDocShellTreeItem> primaryContent;
   if (aCallerType != CallerType::System) {
     if (mDoc) {
@@ -5403,19 +5396,6 @@ nsresult nsGlobalWindowOuter::MoveBy(int32_t aXDif, int32_t aYDif) {
 void nsGlobalWindowOuter::ResizeToOuter(int32_t aWidth, int32_t aHeight,
                                         CallerType aCallerType,
                                         ErrorResult& aError) {
-  /*
-   * If caller is a browser-element then dispatch a resize event to
-   * the embedder.
-   */
-  if (mDocShell && mDocShell->GetIsMozBrowser()) {
-    CSSIntSize size(aWidth, aHeight);
-    if (!DispatchResizeEvent(size)) {
-      // The embedder chose to prevent the default action for this
-      // event, so let's not resize this window after all...
-      return;
-    }
-  }
-
   /*
    * If caller is not chrome and the user has not explicitly exempted the site,
    * prevent window.resizeTo() by exiting early
@@ -5444,26 +5424,6 @@ void nsGlobalWindowOuter::ResizeToOuter(int32_t aWidth, int32_t aHeight,
 void nsGlobalWindowOuter::ResizeByOuter(int32_t aWidthDif, int32_t aHeightDif,
                                         CallerType aCallerType,
                                         ErrorResult& aError) {
-  /*
-   * If caller is a browser-element then dispatch a resize event to
-   * parent.
-   */
-  if (mDocShell && mDocShell->GetIsMozBrowser()) {
-    CSSIntSize size;
-    if (NS_FAILED(GetInnerSize(size))) {
-      return;
-    }
-
-    size.width += aWidthDif;
-    size.height += aHeightDif;
-
-    if (!DispatchResizeEvent(size)) {
-      // The embedder chose to prevent the default action for this
-      // event, so let's not resize this window after all...
-      return;
-    }
-  }
-
   /*
    * If caller is not chrome and the user has not explicitly exempted the site,
    * prevent window.resizeBy() by exiting early
@@ -6131,8 +6091,7 @@ bool nsGlobalWindowOuter::CanClose() {
 }
 
 void nsGlobalWindowOuter::CloseOuter(bool aTrustedCaller) {
-  if (!mDocShell || IsInModalState() ||
-      (IsFrame() && !mDocShell->GetIsMozBrowser())) {
+  if (!mDocShell || IsInModalState() || IsFrame()) {
     // window.close() is called on a frame in a frameset, on a window
     // that's already closed, or on a window for which there's
     // currently a modal dialog open. Ignore such calls.
diff --git a/dom/ipc/BrowserChild.cpp b/dom/ipc/BrowserChild.cpp
index 5437fdb213a8..09f837f89e58 100644
--- a/dom/ipc/BrowserChild.cpp
+++ b/dom/ipc/BrowserChild.cpp
@@ -596,24 +596,12 @@ void BrowserChild::NotifyTabContextUpdated() {
     return;
   }
 
-  UpdateFrameType();
-
   // Set SANDBOXED_AUXILIARY_NAVIGATION flag if this is a receiver page.
   if (!PresentationURL().IsEmpty()) {
     mBrowsingContext->SetSandboxFlags(SANDBOXED_AUXILIARY_NAVIGATION);
   }
 }
 
-void BrowserChild::UpdateFrameType() {
-  nsCOMPtr<nsIDocShell> docShell = do_GetInterface(WebNavigation());
-  MOZ_ASSERT(docShell);
-
-  // TODO: Bug 1252794 - remove frameType from nsIDocShell.idl
-  docShell->SetFrameType(IsMozBrowserElement()
-                             ? nsIDocShell::FRAME_TYPE_BROWSER
-                             : nsIDocShell::FRAME_TYPE_REGULAR);
-}
-
 NS_IMPL_CYCLE_COLLECTION_CLASS(BrowserChild)
 
 NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(BrowserChild)
@@ -1096,9 +1084,6 @@ void BrowserChild::ApplyParentShowInfo(const ParentShowInfo& aInfo) {
     mDidSetRealShowInfo = true;
   }
 
-  if (nsCOMPtr<nsIDocShell> docShell = do_GetInterface(WebNavigation())) {
-    docShell->SetFullscreenAllowed(aInfo.fullscreenAllowed());
-  }
   mIsTransparent = aInfo.isTransparent();
 }
 
@@ -2267,11 +2252,6 @@ mozilla::ipc::IPCResult BrowserChild::RecvSwappedWithOtherRemoteLoader(
     MOZ_CRASH("Update to TabContext after swap was denied.");
   }
 
-  // Since mIsMozBrowserElement may change in UpdateTabContextAfterSwap, so we
-  // call UpdateFrameType here to make sure the frameType on the docshell is
-  // correct.
-  UpdateFrameType();
-
   // Ignore previous value of mTriedBrowserInit since owner content has changed.
   mTriedBrowserInit = true;
   // Initialize the child side of the browser element machinery, if appropriate.
diff --git a/dom/ipc/BrowserChild.h b/dom/ipc/BrowserChild.h
index 8c60efaf49f1..694ecaf9d890 100644
--- a/dom/ipc/BrowserChild.h
+++ b/dom/ipc/BrowserChild.h
@@ -770,9 +770,6 @@ class BrowserChild final : public nsMessageManagerScriptExecutor,
   // call this during Init().
   void NotifyTabContextUpdated();
 
-  // Update the frameType on our docshell.
-  void UpdateFrameType();
-
   void ActorDestroy(ActorDestroyReason why) override;
 
   bool InitBrowserChildMessageManager();
diff --git a/dom/presentation/Presentation.cpp b/dom/presentation/Presentation.cpp
index fe915eb9fb9d..5c0efd7693de 100644
--- a/dom/presentation/Presentation.cpp
+++ b/dom/presentation/Presentation.cpp
@@ -124,7 +124,6 @@ bool Presentation::HasReceiverSupport() const {
   }
 
   if (!StaticPrefs::dom_presentation_testing_simulate_receiver() &&
-      !docShell->GetIsInMozBrowser() &&
       !docShell->GetIsTopLevelContentDocShell()) {
     return false;
   }
diff --git a/toolkit/components/windowwatcher/nsWindowWatcher.cpp b/toolkit/components/windowwatcher/nsWindowWatcher.cpp
index c174643a37a2..2f6825bd0b40 100644
--- a/toolkit/components/windowwatcher/nsWindowWatcher.cpp
+++ b/toolkit/components/windowwatcher/nsWindowWatcher.cpp
@@ -1980,13 +1980,6 @@ uint32_t nsWindowWatcher::CalculateChromeFlagsForParent(
     chromeFlags = EnsureFlagsSafeForContent(chromeFlags, aChromeURL);
   }
 
-  // Disable CHROME_OPENAS_DIALOG if the window is inside <iframe mozbrowser>.
-  // It's up to the embedder to interpret what dialog=1 means.
-  nsCOMPtr<nsIDocShell> docshell = do_GetInterface(aParent);
-  if (docshell && docshell->GetIsInMozBrowser()) {
-    chromeFlags &= ~nsIWebBrowserChrome::CHROME_OPENAS_DIALOG;
-  }
-
   return chromeFlags;
 }