--- a/browser/modules/FormSubmitObserver.jsm
+++ b/browser/modules/FormSubmitObserver.jsm
@@ -8,21 +8,16 @@
  */
 
 "use strict";
 
 var Cc = Components.classes;
 var Ci = Components.interfaces;
 var Cu = Components.utils;
 
-var HTMLInputElement = Ci.nsIDOMHTMLInputElement;
-var HTMLTextAreaElement = Ci.nsIDOMHTMLTextAreaElement;
-var HTMLSelectElement = Ci.nsIDOMHTMLSelectElement;
-var HTMLButtonElement = Ci.nsIDOMHTMLButtonElement;
-
 this.EXPORTED_SYMBOLS = [ "FormSubmitObserver" ];
 
 Cu.import("resource://gre/modules/XPCOMUtils.jsm");
 Cu.import("resource://gre/modules/Services.jsm");
 Cu.import("resource://gre/modules/BrowserUtils.jsm");
 
 function FormSubmitObserver(aWindow, aTabChildGlobal) {
   this.init(aWindow, aTabChildGlobal);
@@ -105,20 +100,20 @@ FormSubmitObserver.prototype =
     for (let i = 0; i < aInvalidElements.length; i++) {
       // Insure that this is the FormSubmitObserver associated with the
       // element / window this notification is about.
       let element = aInvalidElements.queryElementAt(i, Ci.nsISupports);
       if (this._content != element.ownerGlobal.top.document.defaultView) {
         return;
       }
 
-      if (!(element instanceof HTMLInputElement ||
-            element instanceof HTMLTextAreaElement ||
-            element instanceof HTMLSelectElement ||
-            element instanceof HTMLButtonElement)) {
+      if (!(ChromeUtils.getClassName(element) === "HTMLInputElement" ||
+            ChromeUtils.getClassName(element) === "HTMLTextAreaElement" ||
+            ChromeUtils.getClassName(element) === "HTMLSelectElement" ||
+            ChromeUtils.getClassName(element) === "HTMLButtonElement")) {
         continue;
       }
 
       if (!Services.focus.elementIsFocusable(element, 0)) {
         continue;
       }
 
       // Update validation message before showing notification

--- a/dom/html/HTMLButtonElement.cpp
+++ b/dom/html/HTMLButtonElement.cpp
@@ -73,17 +73,16 @@ HTMLButtonElement::~HTMLButtonElement()
 // nsISupports
 
 NS_IMPL_CYCLE_COLLECTION_INHERITED(HTMLButtonElement,
                                    nsGenericHTMLFormElementWithState,
                                    mValidity)
 
 NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED(HTMLButtonElement,
                                              nsGenericHTMLFormElementWithState,
-                                             nsIDOMHTMLButtonElement,
                                              nsIConstraintValidation)
 
 void
 HTMLButtonElement::SetCustomValidity(const nsAString& aError)
 {
   nsIConstraintValidation::SetCustomValidity(aError);
 
   UpdateState(true);
@@ -105,42 +104,35 @@ HTMLButtonElement::FieldSetDisabledChang
   // UpdateBarredFromConstraintValidation, because the latter depends on our
   // disabled state.
   nsGenericHTMLFormElementWithState::FieldSetDisabledChanged(aNotify);
 
   UpdateBarredFromConstraintValidation();
   UpdateState(aNotify);
 }
 
-// nsIDOMHTMLButtonElement
-
 NS_IMPL_ELEMENT_CLONE(HTMLButtonElement)
 
-
-// nsIDOMHTMLButtonElement
-
-NS_IMETHODIMP
-HTMLButtonElement::GetForm(nsIDOMHTMLFormElement** aForm)
+void
+HTMLButtonElement::GetFormEnctype(nsAString& aFormEncType)
 {
-  return nsGenericHTMLFormElementWithState::GetForm(aForm);
+  GetEnumAttr(nsGkAtoms::formenctype, "", kFormDefaultEnctype->tag, aFormEncType);
 }
 
-NS_IMPL_BOOL_ATTR(HTMLButtonElement, Autofocus, autofocus)
-NS_IMPL_BOOL_ATTR(HTMLButtonElement, Disabled, disabled)
-NS_IMPL_ACTION_ATTR(HTMLButtonElement, FormAction, formaction)
-NS_IMPL_ENUM_ATTR_DEFAULT_MISSING_INVALID_VALUES(HTMLButtonElement, FormEnctype, formenctype,
-                                                 "", kFormDefaultEnctype->tag)
-NS_IMPL_ENUM_ATTR_DEFAULT_MISSING_INVALID_VALUES(HTMLButtonElement, FormMethod, formmethod,
-                                                 "", kFormDefaultMethod->tag)
-NS_IMPL_BOOL_ATTR(HTMLButtonElement, FormNoValidate, formnovalidate)
-NS_IMPL_STRING_ATTR(HTMLButtonElement, FormTarget, formtarget)
-NS_IMPL_STRING_ATTR(HTMLButtonElement, Name, name)
-NS_IMPL_STRING_ATTR(HTMLButtonElement, Value, value)
-NS_IMPL_ENUM_ATTR_DEFAULT_VALUE(HTMLButtonElement, Type, type,
-                                kButtonDefaultType->tag)
+void
+HTMLButtonElement::GetFormMethod(nsAString& aFormMethod)
+{
+  GetEnumAttr(nsGkAtoms::formmethod, "", kFormDefaultMethod->tag, aFormMethod);
+}
+
+void
+HTMLButtonElement::GetType(nsAString& aType)
+{
+  GetEnumAttr(nsGkAtoms::type, kButtonDefaultType->tag, aType);
+}
 
 int32_t
 HTMLButtonElement::TabIndexDefault()
 {
   return 0;
 }
 
 bool
@@ -370,29 +362,26 @@ HTMLButtonElement::SubmitNamesValues(HTM
   if (IsDisabled()) {
     return NS_OK;
   }
 
   //
   // Get the name (if no name, no submit)
   //
   nsAutoString name;
-  GetAttr(kNameSpaceID_None, nsGkAtoms::name, name);
+  GetHTMLAttr(nsGkAtoms::name, name);
   if (name.IsEmpty()) {
     return NS_OK;
   }
 
   //
   // Get the value
   //
   nsAutoString value;
-  nsresult rv = GetValue(value);
-  if (NS_FAILED(rv)) {
-    return rv;
-  }
+  GetHTMLAttr(nsGkAtoms::value, value);
 
   //
   // Submit
   //
   return aFormSubmission->AddNameValuePair(name, value);
 }
 
 void
@@ -466,17 +455,18 @@ HTMLButtonElement::SaveState()
 
   return NS_OK;
 }
 
 bool
 HTMLButtonElement::RestoreState(nsPresState* aState)
 {
   if (aState && aState->IsDisabledSet() && !aState->GetDisabled()) {
-    SetDisabled(false);
+    IgnoredErrorResult rv;
+    SetDisabled(false, rv);
   }
 
   return false;
 }
 
 EventStates
 HTMLButtonElement::IntrinsicState() const
 {

--- a/dom/html/HTMLButtonElement.h
+++ b/dom/html/HTMLButtonElement.h
@@ -4,31 +4,28 @@
  * 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/. */
 
 #ifndef mozilla_dom_HTMLButtonElement_h
 #define mozilla_dom_HTMLButtonElement_h
 
 #include "mozilla/Attributes.h"
 #include "nsGenericHTMLElement.h"
-#include "nsIDOMHTMLButtonElement.h"
 #include "nsIConstraintValidation.h"
 
 namespace mozilla {
 class EventChainPostVisitor;
 class EventChainPreVisitor;
 namespace dom {
 
 class HTMLButtonElement final : public nsGenericHTMLFormElementWithState,
-                                public nsIDOMHTMLButtonElement,
                                 public nsIConstraintValidation
 {
 public:
   using nsIConstraintValidation::GetValidationMessage;
-  using nsGenericHTMLFormElementWithState::GetFormAction;
 
   explicit HTMLButtonElement(already_AddRefed<mozilla::dom::NodeInfo>& aNodeInfo,
                              FromParser aFromParser = NOT_FROM_PARSER);
 
   NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(HTMLButtonElement,
                                            nsGenericHTMLFormElementWithState)
 
   // nsISupports
@@ -39,19 +36,16 @@ public:
   NS_IMPL_FROMCONTENT_HTML_WITH_TAG(HTMLButtonElement, button)
 
   // Element
   virtual bool IsInteractiveHTMLContent(bool aIgnoreTabindex) const override
   {
     return true;
   }
 
-  // nsIDOMHTMLButtonElement
-  NS_DECL_NSIDOMHTMLBUTTONELEMENT
-
   // overriden nsIFormControl methods
   NS_IMETHOD Reset() override;
   NS_IMETHOD SubmitNamesValues(HTMLFormSubmission* aFormSubmission) override;
   NS_IMETHOD SaveState() override;
   bool RestoreState(nsPresState* aState) override;
   virtual bool IsDisabledForEvents(EventMessage aMessage) override;
 
   virtual void FieldSetDisabledChanged(bool aNotify) override;
@@ -115,55 +109,64 @@ public:
     return GetBoolAttr(nsGkAtoms::disabled);
   }
   void SetDisabled(bool aDisabled, ErrorResult& aError)
   {
     SetHTMLBoolAttr(nsGkAtoms::disabled, aDisabled, aError);
   }
   // nsGenericHTMLFormElement::GetForm is fine.
   using nsGenericHTMLFormElement::GetForm;
-  // XPCOM GetFormAction is fine.
+  // GetFormAction implemented in superclass
   void SetFormAction(const nsAString& aFormAction, ErrorResult& aRv)
   {
     SetHTMLAttr(nsGkAtoms::formaction, aFormAction, aRv);
   }
-  // XPCOM GetFormEnctype is fine.
+  void GetFormEnctype(nsAString& aFormEncType);
   void SetFormEnctype(const nsAString& aFormEnctype, ErrorResult& aRv)
   {
     SetHTMLAttr(nsGkAtoms::formenctype, aFormEnctype, aRv);
   }
-  // XPCOM GetFormMethod is fine.
+  void GetFormMethod(nsAString& aFormMethod);
   void SetFormMethod(const nsAString& aFormMethod, ErrorResult& aRv)
   {
     SetHTMLAttr(nsGkAtoms::formmethod, aFormMethod, aRv);
   }
   bool FormNoValidate() const
   {
     return GetBoolAttr(nsGkAtoms::formnovalidate);
   }
   void SetFormNoValidate(bool aFormNoValidate, ErrorResult& aError)
   {
     SetHTMLBoolAttr(nsGkAtoms::formnovalidate, aFormNoValidate, aError);
   }
-  // XPCOM GetFormTarget is fine.
+  void GetFormTarget(DOMString& aFormTarget)
+  {
+    GetHTMLAttr(nsGkAtoms::formtarget, aFormTarget);
+  }
   void SetFormTarget(const nsAString& aFormTarget, ErrorResult& aRv)
   {
     SetHTMLAttr(nsGkAtoms::formtarget, aFormTarget, aRv);
   }
-  // XPCOM GetName is fine.
+  void GetName(DOMString& aName)
+  {
+    GetHTMLAttr(nsGkAtoms::name, aName);
+  }
   void SetName(const nsAString& aName, ErrorResult& aRv)
   {
     SetHTMLAttr(nsGkAtoms::name, aName, aRv);
   }
-  // XPCOM GetType is fine.
+  void GetType(nsAString& aType);
   void SetType(const nsAString& aType, ErrorResult& aRv)
   {
     SetHTMLAttr(nsGkAtoms::type, aType, aRv);
   }
-  // XPCOM GetValue is fine.
+  void GetValue(DOMString& aValue)
+  {
+    GetHTMLAttr(nsGkAtoms::value, aValue);
+  }
   void SetValue(const nsAString& aValue, ErrorResult& aRv)
   {
     SetHTMLAttr(nsGkAtoms::value, aValue, aRv);
   }
 
   // Override SetCustomValidity so we update our state properly when it's called
   // via bindings.
   void SetCustomValidity(const nsAString& aError);

--- a/dom/html/HTMLFormElement.cpp
+++ b/dom/html/HTMLFormElement.cpp
@@ -61,23 +61,23 @@
 
 #include "nsLayoutUtils.h"
 
 #include "mozAutoDocUpdate.h"
 #include "nsIHTMLCollection.h"
 
 #include "nsIConstraintValidation.h"
 
-#include "nsIDOMHTMLButtonElement.h"
 #include "nsSandboxFlags.h"
 
 #include "nsIContentSecurityPolicy.h"
 
 // images
 #include "mozilla/dom/HTMLImageElement.h"
+#include "mozilla/dom/HTMLButtonElement.h"
 
 // construction, destruction
 NS_IMPL_NS_NEW_HTML_ELEMENT(Form)
 
 namespace mozilla {
 namespace dom {
 
 static const uint8_t NS_FORM_AUTOCOMPLETE_ON  = 1;
@@ -1644,17 +1644,17 @@ HTMLFormElement::GetActionURL(nsIURI** a
     NS_ASSERTION(formControl && formControl->IsSubmitControl(),
                  "The originating element must be a submit form control!");
 #endif // DEBUG
 
     nsCOMPtr<nsIDOMHTMLInputElement> inputElement = do_QueryInterface(aOriginatingElement);
     if (inputElement) {
       inputElement->GetFormAction(action);
     } else {
-      nsCOMPtr<nsIDOMHTMLButtonElement> buttonElement = do_QueryInterface(aOriginatingElement);
+      auto buttonElement = HTMLButtonElement::FromContent(aOriginatingElement);
       if (buttonElement) {
         buttonElement->GetFormAction(action);
       } else {
         NS_ERROR("Originating element must be an input or button element!");
         return NS_ERROR_UNEXPECTED;
       }
     }
   } else {

--- a/dom/interfaces/html/moz.build
+++ b/dom/interfaces/html/moz.build
@@ -5,17 +5,16 @@
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 with Files("**"):
     BUG_COMPONENT = ("Core", "DOM")
 
 XPIDL_SOURCES += [
     'nsIDOMHTMLAreaElement.idl',
     'nsIDOMHTMLBaseElement.idl',
-    'nsIDOMHTMLButtonElement.idl',
     'nsIDOMHTMLCanvasElement.idl',
     'nsIDOMHTMLCollection.idl',
     'nsIDOMHTMLDocument.idl',
     'nsIDOMHTMLElement.idl',
     'nsIDOMHTMLFormElement.idl',
     'nsIDOMHTMLFrameElement.idl',
     'nsIDOMHTMLHtmlElement.idl',
     'nsIDOMHTMLIFrameElement.idl',

--- a/mobile/android/components/BrowserCLH.js
+++ b/mobile/android/components/BrowserCLH.js
@@ -133,20 +133,20 @@ BrowserCLH.prototype = {
             mozSystemGroup: true,
           },
         });
 
         GeckoViewUtils.addLazyEventListener(win, [
           "focus", "blur", "click", "input",
         ], {
           handler: event => {
-            if (event.target instanceof Ci.nsIDOMHTMLInputElement ||
-                event.target instanceof Ci.nsIDOMHTMLTextAreaElement ||
-                event.target instanceof Ci.nsIDOMHTMLSelectElement ||
-                event.target instanceof Ci.nsIDOMHTMLButtonElement) {
+            if (ChromeUtils.getClassName(event.target) === "HTMLInputElement" ||
+                ChromeUtils.getClassName(event.target) === "HTMLTextAreaElement" ||
+                ChromeUtils.getClassName(event.target) === "HTMLSelectElement" ||
+                ChromeUtils.getClassName(event.target) === "HTMLButtonElement") {
               // Only load FormAssistant when the event target is what we care about.
               return this.FormAssistant;
             }
             return null;
           },
           options: {
             capture: true,
             mozSystemGroup: true,

--- a/mobile/android/modules/FormAssistant.jsm
+++ b/mobile/android/modules/FormAssistant.jsm
@@ -283,20 +283,20 @@ var FormAssistant = {
     };
 
     this._getAutoCompleteSuggestions(aElement.value, aElement, resultsAvailable);
   },
 
   // Only show a validation message if the user submitted an invalid form,
   // there's a non-empty message string, and the element is the correct type
   _isValidateable: function(aElement) {
-    return (aElement instanceof Ci.nsIDOMHTMLInputElement ||
-            aElement instanceof Ci.nsIDOMHTMLTextAreaElement ||
-            aElement instanceof Ci.nsIDOMHTMLSelectElement ||
-            aElement instanceof Ci.nsIDOMHTMLButtonElement) &&
+    return (ChromeUtils.getClassName(aElement) === "HTMLInputElement" ||
+            ChromeUtils.getClassName(aElement) === "HTMLTextAreaElement" ||
+            ChromeUtils.getClassName(aElement) === "HTMLSelectElement" ||
+            ChromeUtils.getClassName(aElement) === "HTMLButtonElement") &&
            aElement.validationMessage;
   },
 
   // Sends a validation message and position data for an element to the Java UI.
   // Returns true if there's a validation message to show, false otherwise.
   _showValidationMessage: function(aElement) {
     if (!this._isValidateable(aElement)) {
       return false;

--- a/xpcom/reflect/xptinfo/ShimInterfaceInfo.cpp
+++ b/xpcom/reflect/xptinfo/ShimInterfaceInfo.cpp
@@ -42,17 +42,16 @@
 #include "nsIDOMEventTarget.h"
 #include "nsIDOMFileList.h"
 #include "nsIDOMFocusEvent.h"
 #include "nsIDOMFormData.h"
 #include "nsIDOMGeoPositionError.h"
 #include "nsIDOMHistory.h"
 #include "nsIDOMHTMLAreaElement.h"
 #include "nsIDOMHTMLBaseElement.h"
-#include "nsIDOMHTMLButtonElement.h"
 #include "nsIDOMHTMLCanvasElement.h"
 #include "nsIDOMHTMLCollection.h"
 #include "nsIDOMHTMLDocument.h"
 #include "nsIDOMHTMLElement.h"
 #include "nsIDOMHTMLFormElement.h"
 #include "nsIDOMHTMLFrameElement.h"
 #include "nsIDOMHTMLHtmlElement.h"
 #include "nsIDOMHTMLIFrameElement.h"
@@ -317,17 +316,16 @@ const ComponentsInterfaceShimEntry kComp
   DEFINE_SHIM(FileList),
   DEFINE_SHIM(FocusEvent),
   DEFINE_SHIM(FormData),
   DEFINE_SHIM_WITH_CUSTOM_INTERFACE(nsIFrameLoader, FrameLoader),
   DEFINE_SHIM_WITH_CUSTOM_INTERFACE(nsIDOMGeoPositionError, PositionError),
   DEFINE_SHIM(History),
   DEFINE_SHIM(HTMLAreaElement),
   DEFINE_SHIM(HTMLBaseElement),
-  DEFINE_SHIM(HTMLButtonElement),
   DEFINE_SHIM(HTMLCanvasElement),
   DEFINE_SHIM(HTMLCollection),
   DEFINE_SHIM(HTMLDocument),
   DEFINE_SHIM(HTMLElement),
   DEFINE_SHIM(HTMLFormElement),
   DEFINE_SHIM(HTMLFrameElement),
   DEFINE_SHIM(HTMLHtmlElement),
   DEFINE_SHIM(HTMLIFrameElement),