diff --git a/dom/base/Element.h b/dom/base/Element.h
index 3641d5fb909a..1692e41b9a3d 100644
--- a/dom/base/Element.h
+++ b/dom/base/Element.h
@@ -90,7 +90,6 @@ class nsIDOMXULSelectControlElement;
 class nsIDOMXULSelectControlItemElement;
 class nsIFrame;
 class nsIHTMLCollection;
-class nsIMozBrowserFrame;
 class nsIPrincipal;
 class nsIScreen;
 class nsIScrollableFrame;
@@ -458,16 +457,6 @@ class Element : public FragmentOrElement {
    */
   virtual bool IsInteractiveHTMLContent() const;
 
-  /**
-   * Returns |this| as an nsIMozBrowserFrame* if the element is a frame or
-   * iframe element.
-   *
-   * We have this method, rather than using QI, so that we can use it during
-   * the servo traversal, where we can't QI DOM nodes because of non-thread-safe
-   * refcounts.
-   */
-  virtual nsIMozBrowserFrame* GetAsMozBrowserFrame() { return nullptr; }
-
   /**
    * Is the attribute named aAttribute a mapped attribute?
    */
diff --git a/dom/base/InProcessBrowserChildMessageManager.cpp b/dom/base/InProcessBrowserChildMessageManager.cpp
index 191ec9f7b52b..1de39cbd98f6 100644
--- a/dom/base/InProcessBrowserChildMessageManager.cpp
+++ b/dom/base/InProcessBrowserChildMessageManager.cpp
@@ -13,7 +13,6 @@
 #include "nsFrameLoaderOwner.h"
 #include "nsQueryObject.h"
 #include "xpcpublic.h"
-#include "nsIMozBrowserFrame.h"
 #include "mozilla/EventDispatcher.h"
 #include "mozilla/dom/ChromeMessageSender.h"
 #include "mozilla/dom/Document.h"
@@ -100,15 +99,6 @@ InProcessBrowserChildMessageManager::InProcessBrowserChildMessageManager(
       mOwner(aOwner),
       mChromeMessageManager(aChrome) {
   mozilla::HoldJSObjects(this);
-
-  // If owner corresponds to an <iframe mozbrowser>, we'll have to tweak our
-  // GetEventTargetParent implementation.
-  nsCOMPtr<nsIMozBrowserFrame> browserFrame = do_QueryInterface(mOwner);
-  if (browserFrame) {
-    mIsBrowserFrame = browserFrame->GetReallyIsBrowser();
-  } else {
-    mIsBrowserFrame = false;
-  }
 }
 
 InProcessBrowserChildMessageManager::~InProcessBrowserChildMessageManager() {
diff --git a/dom/base/nsFrameLoader.cpp b/dom/base/nsFrameLoader.cpp
index a40bc427dda7..4b0189f08a8e 100644
--- a/dom/base/nsFrameLoader.cpp
+++ b/dom/base/nsFrameLoader.cpp
@@ -39,7 +39,6 @@
 #include "nsSubDocumentFrame.h"
 #include "nsError.h"
 #include "nsIAppWindow.h"
-#include "nsIMozBrowserFrame.h"
 #include "nsIScriptError.h"
 #include "nsGlobalWindowInner.h"
 #include "nsGlobalWindowOuter.h"
@@ -262,13 +261,6 @@ static bool IsTopContent(BrowsingContext* aParent, Element* aOwner) {
     return false;
   }
 
-  // If we have a (deprecated) mozbrowser element, we want to start a new
-  // BrowsingContext tree regardless of whether the parent is chrome or content.
-  nsCOMPtr<nsIMozBrowserFrame> mozbrowser = aOwner->GetAsMozBrowserFrame();
-  if (mozbrowser && mozbrowser->GetReallyIsBrowser()) {
-    return true;
-  }
-
   if (aParent->IsContent()) {
     // If we're already in content, we may still want to create a new
     // BrowsingContext tree if our element is a xul browser element with a
@@ -365,17 +357,8 @@ static bool InitialLoadIsRemote(Element* aOwner) {
     return false;
   }
 
-  // If we're an <iframe mozbrowser> and we don't have a "remote" attribute,
-  // fall back to the default.
-  nsCOMPtr<nsIMozBrowserFrame> browserFrame = do_QueryInterface(aOwner);
-  bool isMozBrowserFrame = browserFrame && browserFrame->GetReallyIsBrowser();
-  if (isMozBrowserFrame && !aOwner->HasAttr(nsGkAtoms::remote)) {
-    return Preferences::GetBool("dom.ipc.browser_frames.oop_by_default", false);
-  }
-
-  // Otherwise, we're remote if we have "remote=true" and we're either a
-  // browser frame or a XUL element.
-  return (isMozBrowserFrame || aOwner->GetNameSpaceID() == kNameSpaceID_XUL) &&
+  // Otherwise, we're remote if we have "remote=true" and we're a XUL element.
+  return (aOwner->GetNameSpaceID() == kNameSpaceID_XUL) &&
          aOwner->AttrValueIs(kNameSpaceID_None, nsGkAtoms::remote,
                              nsGkAtoms::_true, eCaseMatters);
 }
@@ -706,12 +689,6 @@ nsresult nsFrameLoader::ReallyStartLoadingInternal() {
 
     // Default flags:
     int32_t flags = nsIWebNavigation::LOAD_FLAGS_NONE;
-
-    // Flags for browser frame:
-    if (OwnerIsMozBrowserFrame()) {
-      flags = nsIWebNavigation::LOAD_FLAGS_ALLOW_THIRD_PARTY_FIXUP |
-              nsIWebNavigation::LOAD_FLAGS_DISALLOW_INHERIT_PRINCIPAL;
-    }
     loadState->SetLoadFlags(flags);
 
     loadState->SetFirstParty(false);
@@ -875,14 +852,6 @@ static bool CheckDocShellType(mozilla::dom::Element* aOwnerContent,
   bool isContent = aOwnerContent->AttrValueIs(kNameSpaceID_None, aAtom,
                                               nsGkAtoms::content, eIgnoreCase);
 
-  if (!isContent) {
-    nsCOMPtr<nsIMozBrowserFrame> mozbrowser =
-        aOwnerContent->GetAsMozBrowserFrame();
-    if (mozbrowser) {
-      mozbrowser->GetMozbrowser(&isContent);
-    }
-  }
-
   if (isContent) {
     return aDocShell->ItemType() == nsIDocShellTreeItem::typeContent;
   }
@@ -1156,7 +1125,6 @@ bool nsFrameLoader::ShowRemoteFrame(const ScreenIntSize& size,
       if (nsCOMPtr<nsIObserverService> os = services::GetObserverService()) {
         os->NotifyObservers(ToSupports(this), "remote-browser-shown", nullptr);
       }
-      ProcessPriorityManager::RemoteBrowserFrameShown(this);
     }
   } else {
     nsIntRect dimensions;
@@ -1329,14 +1297,6 @@ nsresult nsFrameLoader::SwapWithOtherRemoteLoader(
     return NS_ERROR_NOT_IMPLEMENTED;
   }
 
-  // Destroy browser frame scripts for content leaving a frame with browser API
-  if (OwnerIsMozBrowserFrame() && !aOther->OwnerIsMozBrowserFrame()) {
-    DestroyBrowserFrameScripts();
-  }
-  if (!OwnerIsMozBrowserFrame() && aOther->OwnerIsMozBrowserFrame()) {
-    aOther->DestroyBrowserFrameScripts();
-  }
-
   otherBrowserParent->SetBrowserDOMWindow(browserDOMWindow);
   browserParent->SetBrowserDOMWindow(otherBrowserDOMWindow);
 
@@ -1405,10 +1365,6 @@ nsresult nsFrameLoader::SwapWithOtherRemoteLoader(
   ourPresShell->BackingScaleFactorChanged();
   otherPresShell->BackingScaleFactorChanged();
 
-  // Initialize browser API if needed now that owner content has changed.
-  InitializeBrowserAPI();
-  aOther->InitializeBrowserAPI();
-
   mInSwap = aOther->mInSwap = false;
 
   // Send an updated tab context since owner content type may have changed.
@@ -1536,13 +1492,8 @@ nsresult nsFrameLoader::SwapWithOtherLoader(nsFrameLoader* aOther,
     return NS_ERROR_NOT_IMPLEMENTED;
   }
 
-  bool ourFullscreenAllowed = ourContent->IsXULElement() ||
-                              (OwnerIsMozBrowserFrame() &&
-                               ourContent->HasAttr(nsGkAtoms::allowfullscreen));
-  bool otherFullscreenAllowed =
-      otherContent->IsXULElement() ||
-      (aOther->OwnerIsMozBrowserFrame() &&
-       otherContent->HasAttr(nsGkAtoms::allowfullscreen));
+  bool ourFullscreenAllowed = ourContent->IsXULElement();
+  bool otherFullscreenAllowed = otherContent->IsXULElement();
   if (ourFullscreenAllowed != otherFullscreenAllowed) {
     return NS_ERROR_NOT_IMPLEMENTED;
   }
@@ -1732,14 +1683,6 @@ nsresult nsFrameLoader::SwapWithOtherLoader(nsFrameLoader* aOther,
     return rv;
   }
 
-  // Destroy browser frame scripts for content leaving a frame with browser API
-  if (OwnerIsMozBrowserFrame() && !aOther->OwnerIsMozBrowserFrame()) {
-    DestroyBrowserFrameScripts();
-  }
-  if (!OwnerIsMozBrowserFrame() && aOther->OwnerIsMozBrowserFrame()) {
-    aOther->DestroyBrowserFrameScripts();
-  }
-
   // Now move the docshells to the right docshell trees.  Note that this
   // resets their treeowners to null.
   ourParentItem->RemoveChild(ourDocshell);
@@ -1837,10 +1780,6 @@ nsresult nsFrameLoader::SwapWithOtherLoader(nsFrameLoader* aOther,
   ourFrame->PresShell()->BackingScaleFactorChanged();
   otherFrame->PresShell()->BackingScaleFactorChanged();
 
-  // Initialize browser API if needed now that owner content has changed
-  InitializeBrowserAPI();
-  aOther->InitializeBrowserAPI();
-
   return NS_OK;
 }
 
@@ -2174,11 +2113,6 @@ void nsFrameLoader::SetOwnerContent(Element* aContent) {
   }
 }
 
-bool nsFrameLoader::OwnerIsMozBrowserFrame() {
-  nsCOMPtr<nsIMozBrowserFrame> browserFrame = do_QueryInterface(mOwnerContent);
-  return browserFrame ? browserFrame->GetReallyIsBrowser() : false;
-}
-
 nsIContent* nsFrameLoader::GetParentObject() const { return mOwnerContent; }
 
 void nsFrameLoader::AssertSafeToInit() {
@@ -2338,16 +2272,7 @@ nsresult nsFrameLoader::MaybeCreateDocShell() {
   MOZ_ALWAYS_SUCCEEDS(mPendingBrowsingContext->SetInitialSandboxFlags(
       mPendingBrowsingContext->GetSandboxFlags()));
 
-  if (OwnerIsMozBrowserFrame()) {
-    // For inproc frames, set the docshell properties.
-    nsAutoString name;
-    if (mOwnerContent->GetAttr(nsGkAtoms::name, name)) {
-      docShell->SetName(name);
-    }
-  }
-
   ReallyLoadFrameScripts();
-  InitializeBrowserAPI();
 
   // Previously we would forcibly create the initial about:blank document for
   // in-process content frames from a frame script which eagerly loaded in
@@ -2588,11 +2513,8 @@ bool nsFrameLoader::TryRemoteBrowserInternal() {
   // Graphics initialization code relies on having a frame for the
   // remote browser case, as we can be inside a popup, which is a different
   // widget.
-  //
-  // FIXME: Ideally this should be unconditional, but we skip if for <iframe
-  // mozbrowser> because the old RDM ui depends on current behavior, and the
-  // mozbrowser frame code is scheduled for deletion, see bug 1574886.
-  if (!OwnerIsMozBrowserFrame() && !mOwnerContent->GetPrimaryFrame()) {
+
+  if (!mOwnerContent->GetPrimaryFrame()) {
     doc->FlushPendingNotifications(FlushType::Frames);
   }
 
@@ -2647,12 +2569,11 @@ bool nsFrameLoader::TryRemoteBrowserInternal() {
     mPendingBrowsingContext->InitSessionHistory();
   }
 
-  // <iframe mozbrowser> gets to skip these checks.
   // iframes for JS plugins also get to skip these checks. We control the URL
   // that gets loaded, but the load is triggered from the document containing
   // the plugin.
   // out of process iframes also get to skip this check.
-  if (!OwnerIsMozBrowserFrame() && !XRE_IsContentProcess()) {
+  if (!XRE_IsContentProcess()) {
     if (parentDocShell->ItemType() != nsIDocShellTreeItem::typeChrome) {
       // Allow three exceptions to this rule :
       // - about:addons so it can load remote extension options pages
@@ -2817,7 +2738,6 @@ bool nsFrameLoader::TryRemoteBrowserInternal() {
   }
 
   ReallyLoadFrameScripts();
-  InitializeBrowserAPI();
 
   return true;
 }
@@ -3053,7 +2973,7 @@ nsresult nsFrameLoader::EnsureMessageManager() {
     return NS_OK;
   }
 
-  if (!mIsTopLevelContent && !OwnerIsMozBrowserFrame() && !IsRemoteFrame() &&
+  if (!mIsTopLevelContent && !IsRemoteFrame() &&
       !(mOwnerContent->IsXULElement() &&
         mOwnerContent->AttrValueIs(kNameSpaceID_None,
                                    nsGkAtoms::forcemessagemanager,
@@ -3538,36 +3458,6 @@ BrowsingContext* nsFrameLoader::GetExtantBrowsingContext() {
   return mPendingBrowsingContext;
 }
 
-void nsFrameLoader::InitializeBrowserAPI() {
-  if (!OwnerIsMozBrowserFrame()) {
-    return;
-  }
-
-  nsresult rv = EnsureMessageManager();
-  if (NS_WARN_IF(NS_FAILED(rv))) {
-    return;
-  }
-  mMessageManager->LoadFrameScript(
-      u"chrome://global/content/BrowserElementChild.js"_ns,
-      /* allowDelayedLoad = */ true,
-      /* aRunInGlobalScope */ true, IgnoreErrors());
-
-  nsCOMPtr<nsIMozBrowserFrame> browserFrame = do_QueryInterface(mOwnerContent);
-  if (browserFrame) {
-    browserFrame->InitializeBrowserAPI();
-  }
-}
-
-void nsFrameLoader::DestroyBrowserFrameScripts() {
-  if (!OwnerIsMozBrowserFrame()) {
-    return;
-  }
-  nsCOMPtr<nsIMozBrowserFrame> browserFrame = do_QueryInterface(mOwnerContent);
-  if (browserFrame) {
-    browserFrame->DestroyBrowserFrameScripts();
-  }
-}
-
 void nsFrameLoader::StartPersistence(
     BrowsingContext* aContext, nsIWebBrowserPersistDocumentReceiver* aRecv,
     ErrorResult& aRv) {
@@ -3673,9 +3563,9 @@ nsresult nsFrameLoader::GetNewTabContext(MutableTabContext* aTabContext,
 
 nsresult nsFrameLoader::PopulateOriginContextIdsFromAttributes(
     OriginAttributes& aAttr) {
-  // Only XUL or mozbrowser frames are allowed to set context IDs
+  // Only XUL are allowed to set context IDs
   uint32_t namespaceID = mOwnerContent->GetNameSpaceID();
-  if (namespaceID != kNameSpaceID_XUL && !OwnerIsMozBrowserFrame()) {
+  if (namespaceID != kNameSpaceID_XUL) {
     return NS_OK;
   }
 
@@ -3899,8 +3789,7 @@ bool nsFrameLoader::EnsureBrowsingContextAttached() {
     // Inherit the `mFirstPartyDomain` flag from our parent document's result
     // principal, if it was set.
     if (parentContext->IsContent() &&
-        !parentDoc->NodePrincipal()->IsSystemPrincipal() &&
-        !OwnerIsMozBrowserFrame()) {
+        !parentDoc->NodePrincipal()->IsSystemPrincipal()) {
       OriginAttributes docAttrs =
           parentDoc->NodePrincipal()->OriginAttributesRef();
       // We only want to inherit firstPartyDomain here, other attributes should
@@ -3918,15 +3807,6 @@ bool nsFrameLoader::EnsureBrowsingContextAttached() {
     if (NS_WARN_IF(NS_FAILED(rv))) {
       return false;
     }
-
-    // <iframe mozbrowser> is allowed to set `mozprivatebrowsing` to
-    // force-enable private browsing.
-    if (OwnerIsMozBrowserFrame()) {
-      if (mOwnerContent->HasAttr(nsGkAtoms::mozprivatebrowsing)) {
-        attrs.SyncAttributesWithPrivateBrowsing(true);
-        usePrivateBrowsing = true;
-      }
-    }
   }
 
   // If we've already been attached, return.
diff --git a/dom/base/nsFrameLoader.h b/dom/base/nsFrameLoader.h
index 159e3865a611..33c8000868b9 100644
--- a/dom/base/nsFrameLoader.h
+++ b/dom/base/nsFrameLoader.h
@@ -264,12 +264,6 @@ class nsFrameLoader final : public nsStubMutationObserver,
 
   bool IsNetworkCreated() const { return mNetworkCreated; }
 
-  /**
-   * Is this a frame loader for a bona fide <iframe mozbrowser>?
-   * <xul:browser> is not a mozbrowser, so this is false for that case.
-   */
-  bool OwnerIsMozBrowserFrame();
-
   nsIContent* GetParentObject() const;
 
   /**
@@ -481,9 +475,6 @@ class nsFrameLoader final : public nsStubMutationObserver,
   void AddTreeItemToTreeOwner(nsIDocShellTreeItem* aItem,
                               nsIDocShellTreeOwner* aOwner);
 
-  void InitializeBrowserAPI();
-  void DestroyBrowserFrameScripts();
-
   nsresult GetNewTabContext(mozilla::dom::MutableTabContext* aTabContext,
                             nsIURI* aURI = nullptr);
 
diff --git a/dom/chrome-webidl/FrameLoader.webidl b/dom/chrome-webidl/FrameLoader.webidl
index df3d03f2b7a3..e05d5b236b8a 100644
--- a/dom/chrome-webidl/FrameLoader.webidl
+++ b/dom/chrome-webidl/FrameLoader.webidl
@@ -122,13 +122,6 @@ interface FrameLoader {
   [Pure]
   readonly attribute unsigned long long childID;
 
-  /**
-   * Find out whether the owner content really is a mozbrowser. <xul:browser>
-   * is not considered to be a mozbrowser frame.
-   */
-  [Pure]
-  readonly attribute boolean ownerIsMozBrowserFrame;
-
   /**
    * The last known width of the frame. Reading this property will not trigger
    * a reflow, and therefore may not reflect the current state of things. It
diff --git a/dom/html/HTMLIFrameElement.h b/dom/html/HTMLIFrameElement.h
index 8a87c024727e..3124e493b5a7 100644
--- a/dom/html/HTMLIFrameElement.h
+++ b/dom/html/HTMLIFrameElement.h
@@ -142,11 +142,6 @@ class HTMLIFrameElement final : public nsGenericHTMLFrameElement {
   Document* GetSVGDocument(nsIPrincipal& aSubjectPrincipal) {
     return GetContentDocument(aSubjectPrincipal);
   }
-  bool Mozbrowser() const { return GetBoolAttr(nsGkAtoms::mozbrowser); }
-  void SetMozbrowser(bool aAllow, ErrorResult& aError) {
-    SetHTMLBoolAttr(nsGkAtoms::mozbrowser, aAllow, aError);
-  }
-  using nsGenericHTMLFrameElement::SetMozbrowser;
   // nsGenericHTMLFrameElement::GetFrameLoader is fine
   // nsGenericHTMLFrameElement::GetAppManifestURL is fine
 
diff --git a/dom/html/nsGenericHTMLFrameElement.cpp b/dom/html/nsGenericHTMLFrameElement.cpp
index 8d07802c1763..92591d7b72ee 100644
--- a/dom/html/nsGenericHTMLFrameElement.cpp
+++ b/dom/html/nsGenericHTMLFrameElement.cpp
@@ -46,19 +46,10 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN_INHERITED(nsGenericHTMLFrameElement,
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mFrameLoader)
 NS_IMPL_CYCLE_COLLECTION_UNLINK_END
 
-NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED(
-    nsGenericHTMLFrameElement, nsGenericHTMLElement, nsFrameLoaderOwner,
-    nsIDOMMozBrowserFrame, nsIMozBrowserFrame, nsGenericHTMLFrameElement)
-
-NS_IMETHODIMP
-nsGenericHTMLFrameElement::GetMozbrowser(bool* aValue) {
-  *aValue = GetBoolAttr(nsGkAtoms::mozbrowser);
-  return NS_OK;
-}
-NS_IMETHODIMP
-nsGenericHTMLFrameElement::SetMozbrowser(bool aValue) {
-  return SetBoolAttr(nsGkAtoms::mozbrowser, aValue);
-}
+NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED(nsGenericHTMLFrameElement,
+                                             nsGenericHTMLElement,
+                                             nsFrameLoaderOwner,
+                                             nsGenericHTMLFrameElement)
 
 int32_t nsGenericHTMLFrameElement::TabIndexDefault() { return 0; }
 
@@ -258,9 +249,6 @@ void nsGenericHTMLFrameElement::AfterSetAttr(
           child->SendScrollbarPreferenceChanged(pref);
         }
       }
-    } else if (aName == nsGkAtoms::mozbrowser) {
-      mReallyIsBrowser = !!aValue && XRE_IsParentProcess() &&
-                         NodePrincipal()->IsSystemPrincipal();
     }
   }
 
@@ -334,26 +322,3 @@ bool nsGenericHTMLFrameElement::IsHTMLFocusable(bool aWithMouse,
   *aIsFocusable = true;
   return false;
 }
-
-/**
- * Return true if this frame element really is a mozbrowser.  (It
- * needs to have the right attributes, and its creator must have the right
- * permissions.)
- */
-/* [infallible] */
-nsresult nsGenericHTMLFrameElement::GetReallyIsBrowser(bool* aOut) {
-  *aOut = mReallyIsBrowser;
-  return NS_OK;
-}
-
-NS_IMETHODIMP
-nsGenericHTMLFrameElement::InitializeBrowserAPI() {
-  MOZ_ASSERT(mFrameLoader);
-  return NS_OK;
-}
-
-NS_IMETHODIMP
-nsGenericHTMLFrameElement::DestroyBrowserFrameScripts() {
-  MOZ_ASSERT(mFrameLoader);
-  return NS_OK;
-}
diff --git a/dom/html/nsGenericHTMLFrameElement.h b/dom/html/nsGenericHTMLFrameElement.h
index 70baf08be470..587e861b88e9 100644
--- a/dom/html/nsGenericHTMLFrameElement.h
+++ b/dom/html/nsGenericHTMLFrameElement.h
@@ -12,7 +12,6 @@
 #include "nsFrameLoader.h"
 #include "nsFrameLoaderOwner.h"
 #include "nsGenericHTMLElement.h"
-#include "nsIMozBrowserFrame.h"
 
 namespace mozilla {
 class ErrorResult;
@@ -37,23 +36,17 @@ class XULFrameElement;
  * A helper class for frame elements
  */
 class nsGenericHTMLFrameElement : public nsGenericHTMLElement,
-                                  public nsFrameLoaderOwner,
-                                  public nsIMozBrowserFrame {
+                                  public nsFrameLoaderOwner {
  public:
   nsGenericHTMLFrameElement(
       already_AddRefed<mozilla::dom::NodeInfo>&& aNodeInfo,
       mozilla::dom::FromParser aFromParser)
       : nsGenericHTMLElement(std::move(aNodeInfo)),
         mSrcLoadHappened(false),
-        mNetworkCreated(aFromParser == mozilla::dom::FROM_PARSER_NETWORK),
-        mBrowserFrameListenersRegistered(false),
-        mReallyIsBrowser(false) {}
+        mNetworkCreated(aFromParser == mozilla::dom::FROM_PARSER_NETWORK) {}
 
   NS_DECL_ISUPPORTS_INHERITED
 
-  NS_DECL_NSIDOMMOZBROWSERFRAME
-  NS_DECL_NSIMOZBROWSERFRAME
-
   NS_DECLARE_STATIC_IID_ACCESSOR(NS_GENERICHTMLFRAMEELEMENT_IID)
 
   // nsIContent
@@ -67,8 +60,6 @@ class nsGenericHTMLFrameElement : public nsGenericHTMLElement,
 
   virtual int32_t TabIndexDefault() override;
 
-  virtual nsIMozBrowserFrame* GetAsMozBrowserFrame() override { return this; }
-
   NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(nsGenericHTMLFrameElement,
                                            nsGenericHTMLElement)
 
@@ -125,9 +116,6 @@ class nsGenericHTMLFrameElement : public nsGenericHTMLElement,
    */
   bool mNetworkCreated;
 
-  bool mBrowserFrameListenersRegistered;
-  bool mReallyIsBrowser;
-
   // This flag is only used by <iframe>. See HTMLIFrameElement::
   // FullscreenFlag() for details. It is placed here so that we
   // do not bloat any struct.
diff --git a/dom/interfaces/html/moz.build b/dom/interfaces/html/moz.build
index 516bce30e203..4c7e0858270d 100644
--- a/dom/interfaces/html/moz.build
+++ b/dom/interfaces/html/moz.build
@@ -9,7 +9,6 @@ with Files("**"):
 
 XPIDL_SOURCES += [
     "nsIDOMMozBrowserFrame.idl",
-    "nsIMozBrowserFrame.idl",
 ]
 
 XPIDL_MODULE = "dom_html"
diff --git a/dom/interfaces/html/nsIMozBrowserFrame.idl b/dom/interfaces/html/nsIMozBrowserFrame.idl
deleted file mode 100644
index aec4fdfbeba5..000000000000
--- a/dom/interfaces/html/nsIMozBrowserFrame.idl
+++ /dev/null
@@ -1,34 +0,0 @@
-/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
-/* vim:set tw=80 expandtab softtabstop=2 ts=2 sw=2: */
-
-/* This Source Code Form is subject to the terms of the Mozilla Public
- * License, v. 2.0. If a copy of the MPL was not distributed with this file,
- * You can obtain one at http://mozilla.org/MPL/2.0/. */
-
-#include "nsIDOMMozBrowserFrame.idl"
-
-interface nsIRemoteTab;
-
-[scriptable, builtinclass, uuid(0c0a862c-1a47-43c0-ae9e-d51835e3e1a6)]
-interface nsIMozBrowserFrame : nsIDOMMozBrowserFrame
-{
-  /**
-   * Gets whether this frame really is a browser frame.
-   *
-   * In order to really be a browser frame, this frame's
-   * nsIDOMMozBrowserFrame::mozbrowser attribute must be true, and the frame
-   * may have to pass various security checks.
-   */
-  [infallible] readonly attribute boolean reallyIsBrowser;
-
-  /**
-   * Initialize the API, and add frame message listener that supports API
-   * invocations.
-   */
-  [noscript] void initializeBrowserAPI();
-
-  /**
-   * Notify frame scripts that support the API to destroy.
-   */
-  [noscript] void destroyBrowserFrameScripts();
-};
diff --git a/dom/ipc/ContentParent.cpp b/dom/ipc/ContentParent.cpp
index 13206216f3bb..8ae81886d723 100644
--- a/dom/ipc/ContentParent.cpp
+++ b/dom/ipc/ContentParent.cpp
@@ -213,7 +213,6 @@
 #include "nsILocalStorageManager.h"
 #include "nsIMemoryInfoDumper.h"
 #include "nsIMemoryReporter.h"
-#include "nsIMozBrowserFrame.h"
 #include "nsINetworkLinkService.h"
 #include "nsIObserverService.h"
 #include "nsIParentChannel.h"
diff --git a/dom/ipc/ProcessPriorityManager.cpp b/dom/ipc/ProcessPriorityManager.cpp
index 2c54b432952b..84779f78a13c 100644
--- a/dom/ipc/ProcessPriorityManager.cpp
+++ b/dom/ipc/ProcessPriorityManager.cpp
@@ -1042,26 +1042,4 @@ void ProcessPriorityManager::BrowserPriorityChanged(
   singleton->BrowserPriorityChanged(aBrowserParent, aPriority);
 }
 
-/* static */
-void ProcessPriorityManager::RemoteBrowserFrameShown(
-    nsFrameLoader* aFrameLoader) {
-  ProcessPriorityManagerImpl* singleton =
-      ProcessPriorityManagerImpl::GetSingleton();
-  if (!singleton) {
-    return;
-  }
-
-  BrowserParent* bp = BrowserParent::GetFrom(aFrameLoader);
-  NS_ENSURE_TRUE_VOID(bp);
-
-  MOZ_ASSERT(XRE_IsParentProcess());
-
-  // Ignore calls that aren't from a Browser.
-  if (!aFrameLoader->OwnerIsMozBrowserFrame()) {
-    return;
-  }
-
-  singleton->ResetPriority(bp->Manager());
-}
-
 }  // namespace mozilla
diff --git a/dom/ipc/ProcessPriorityManager.h b/dom/ipc/ProcessPriorityManager.h
index 8153a609935a..be1508bfbdff 100644
--- a/dom/ipc/ProcessPriorityManager.h
+++ b/dom/ipc/ProcessPriorityManager.h
@@ -80,8 +80,6 @@ class ProcessPriorityManager final {
   static void BrowserPriorityChanged(dom::BrowserParent* aBrowserParent,
                                      bool aPriority);
 
-  static void RemoteBrowserFrameShown(nsFrameLoader* aFrameLoader);
-
  private:
   ProcessPriorityManager();
   ProcessPriorityManager(const ProcessPriorityManager&) = delete;
diff --git a/layout/generic/ViewportFrame.cpp b/layout/generic/ViewportFrame.cpp
index 1837441e86b7..812aaa8e3322 100644
--- a/layout/generic/ViewportFrame.cpp
+++ b/layout/generic/ViewportFrame.cpp
@@ -21,7 +21,6 @@
 #include "nsCanvasFrame.h"
 #include "nsLayoutUtils.h"
 #include "nsSubDocumentFrame.h"
-#include "nsIMozBrowserFrame.h"
 #include "nsPlaceholderFrame.h"
 #include "MobileViewportManager.h"
 
diff --git a/layout/style/GeckoBindings.cpp b/layout/style/GeckoBindings.cpp
index 5c7992f8db76..656d61740cd8 100644
--- a/layout/style/GeckoBindings.cpp
+++ b/layout/style/GeckoBindings.cpp
@@ -27,7 +27,6 @@
 #include "mozilla/dom/DocumentInlines.h"
 #include "nsILoadContext.h"
 #include "nsIFrame.h"
-#include "nsIMozBrowserFrame.h"
 #include "nsINode.h"
 #include "nsIURI.h"
 #include "nsFontMetrics.h"