--- a/browser/modules/FormSubmitObserver.jsm
+++ b/browser/modules/FormSubmitObserver.jsm
@@ -74,28 +74,20 @@ FormSubmitObserver.prototype =
     }
   },
 
   /*
    * nsIFormSubmitObserver
    */
 
   notifyInvalidSubmit(aFormElement, aInvalidElements) {
-    // We are going to handle invalid form submission attempt by focusing the
-    // first invalid element and show the corresponding validation message in a
-    // panel attached to the element.
-    if (!aInvalidElements.length) {
-      return;
-    }
-
     // Show a validation message on the first focusable element.
-    for (let i = 0; i < aInvalidElements.length; i++) {
+    for (let element of aInvalidElements) {
       // 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 (!(ChromeUtils.getClassName(element) === "HTMLInputElement" ||
             ChromeUtils.getClassName(element) === "HTMLTextAreaElement" ||
             ChromeUtils.getClassName(element) === "HTMLSelectElement" ||
             ChromeUtils.getClassName(element) === "HTMLButtonElement")) {

--- a/dom/html/HTMLFormElement.cpp
+++ b/dom/html/HTMLFormElement.cpp
@@ -1821,17 +1821,17 @@ HTMLFormElement::ForgetCurrentSubmission
   nsCOMPtr<nsIWebProgress> webProgress = do_QueryReferent(mWebProgress);
   if (webProgress) {
     webProgress->RemoveProgressListener(this);
   }
   mWebProgress = nullptr;
 }
 
 bool
-HTMLFormElement::CheckFormValidity(nsIMutableArray* aInvalidElements) const
+HTMLFormElement::CheckFormValidity(nsTArray<RefPtr<Element>>* aInvalidElements) const
 {
   bool ret = true;
 
   // This shouldn't be called recursively, so use a rather large value
   // for the preallocated buffer.
   AutoTArray<RefPtr<nsGenericHTMLFormElement>, 100> sortedControls;
   if (NS_FAILED(mControls->GetSortedControls(sortedControls))) {
     return false;
@@ -1849,17 +1849,17 @@ HTMLFormElement::CheckFormValidity(nsIMu
                                            static_cast<nsIContent*>(sortedControls[i]),
                                            NS_LITERAL_STRING("invalid"),
                                            CanBubble::eNo, Cancelable::eYes,
                                            &defaultAction);
 
       // Add all unhandled invalid controls to aInvalidElements if the caller
       // requested them.
       if (defaultAction && aInvalidElements) {
-        aInvalidElements->AppendElement(ToSupports(sortedControls[i]));
+        aInvalidElements->AppendElement(sortedControls[i]);
       }
     }
   }
 
   return ret;
 }
 
 bool
@@ -1898,22 +1898,19 @@ HTMLFormElement::CheckValidFormSubmissio
   NS_ENSURE_SUCCESS(rv, true);
 
   bool hasObserver = false;
   rv = theEnum->HasMoreElements(&hasObserver);
 
   // Do not check form validity if there is no observer for
   // NS_INVALIDFORMSUBMIT_SUBJECT.
   if (NS_SUCCEEDED(rv) && hasObserver) {
-    nsCOMPtr<nsIMutableArray> invalidElements =
-      do_CreateInstance(NS_ARRAY_CONTRACTID, &rv);
-    // Return true on error here because that's what we always did
-    NS_ENSURE_SUCCESS(rv, true);
+    AutoTArray<RefPtr<Element>, 32> invalidElements;
 
-    if (!CheckFormValidity(invalidElements.get())) {
+    if (!CheckFormValidity(&invalidElements)) {
       // For the first invalid submission, we should update element states.
       // We have to do that _before_ calling the observers so we are sure they
       // will not interfere (like focusing the element).
       if (!mEverTriedInvalidSubmit) {
         mEverTriedInvalidSubmit = true;
 
         /*
          * We are going to call update states assuming elements want to
@@ -1953,18 +1950,17 @@ HTMLFormElement::CheckValidFormSubmissio
       nsCOMPtr<nsISupports> inst;
       nsCOMPtr<nsIFormSubmitObserver> observer;
       bool more = true;
       while (NS_SUCCEEDED(theEnum->HasMoreElements(&more)) && more) {
         theEnum->GetNext(getter_AddRefs(inst));
         observer = do_QueryInterface(inst);
 
         if (observer) {
-          observer->NotifyInvalidSubmit(this,
-                                        static_cast<nsIArray*>(invalidElements));
+          observer->NotifyInvalidSubmit(this, invalidElements);
         }
       }
 
       // The form is invalid. Observers have been alerted. Do not submit.
       return false;
     }
   } else {
     NS_WARNING("There is no observer for \"invalidformsubmit\". \

--- a/dom/html/HTMLFormElement.h
+++ b/dom/html/HTMLFormElement.h
@@ -512,17 +512,17 @@ protected:
    * Check the form validity following this algorithm:
    * http://www.whatwg.org/specs/web-apps/current-work/#statically-validate-the-constraints
    *
    * @param aInvalidElements [out] parameter containing the list of unhandled
    * invalid controls.
    *
    * @return Whether the form is currently valid.
    */
-  bool CheckFormValidity(nsIMutableArray* aInvalidElements) const;
+  bool CheckFormValidity(nsTArray<RefPtr<Element>>* aInvalidElements) const;
 
   // Clear the mImageNameLookupTable and mImageElements.
   void Clear();
 
   // Insert a element into the past names map.
   void AddToPastNamesMap(const nsAString& aName, nsISupports* aChild);
 
   // Remove the given element from the past names map.  The element must be an

--- a/dom/html/nsIConstraintValidation.cpp
+++ b/dom/html/nsIConstraintValidation.cpp
@@ -140,16 +140,19 @@ nsIConstraintValidation::ReportValidity(
                                        NS_LITERAL_STRING("invalid"),
                                        CanBubble::eNo,
                                        Cancelable::eYes,
                                        &defaultAction);
   if (!defaultAction) {
     return false;
   }
 
+  AutoTArray<RefPtr<Element>, 1> invalidElements;
+  invalidElements.AppendElement(element);
+
   nsCOMPtr<nsIObserverService> service =
     mozilla::services::GetObserverService();
   if (!service) {
     NS_WARNING("No observer service available!");
     return true;
   }
 
   nsCOMPtr<nsISimpleEnumerator> theEnum;
@@ -157,20 +160,16 @@ nsIConstraintValidation::ReportValidity(
                                             getter_AddRefs(theEnum));
 
   // Return true on error here because that's what we always did
   NS_ENSURE_SUCCESS(rv, true);
 
   bool hasObserver = false;
   rv = theEnum->HasMoreElements(&hasObserver);
 
-  nsCOMPtr<nsIMutableArray> invalidElements =
-    do_CreateInstance(NS_ARRAY_CONTRACTID, &rv);
-  invalidElements->AppendElement(element);
-
   NS_ENSURE_SUCCESS(rv, true);
   nsCOMPtr<nsISupports> inst;
   nsCOMPtr<nsIFormSubmitObserver> observer;
   bool more = true;
   while (NS_SUCCEEDED(theEnum->HasMoreElements(&more)) && more) {
     theEnum->GetNext(getter_AddRefs(inst));
     observer = do_QueryInterface(inst);
 

--- a/dom/html/nsIFormSubmitObserver.idl
+++ b/dom/html/nsIFormSubmitObserver.idl
@@ -6,24 +6,25 @@
 
 #include "nsISupports.idl"
 
 interface mozIDOMWindow;
 interface nsIURI;
 interface nsIArray;
 
 webidl HTMLFormElement;
+webidl Element;
 
 [scriptable, uuid(867cb7e7-835d-408b-9788-d2834d284e03)]
 interface nsIFormSubmitObserver: nsISupports
 {
   void notify(in HTMLFormElement formNode, in mozIDOMWindow window, in nsIURI actionURL, out boolean cancelSubmit);
 
   void notifyInvalidSubmit(in HTMLFormElement formNode,
-                           in nsIArray invalidElements);
+                           in Array<Element> invalidElements);
 };
 
 %{C++
 #define NS_FORMSUBMIT_SUBJECT "formsubmit"
 #define NS_EARLYFORMSUBMIT_SUBJECT "earlyformsubmit"
 #define NS_FIRST_FORMSUBMIT_CATEGORY "firstformsubmit"
 #define NS_PASSWORDMANAGER_CATEGORY "passwordmanager"
 #define NS_INVALIDFORMSUBMIT_SUBJECT "invalidformsubmit"

--- a/mobile/android/modules/FormAssistant.jsm
+++ b/mobile/android/modules/FormAssistant.jsm
@@ -89,17 +89,17 @@ var FormAssistant = {
   },
 
   notifyInvalidSubmit: function(aFormElement, aInvalidElements) {
     if (!aInvalidElements.length) {
         return;
     }
 
     // Ignore this notificaiton if the current tab doesn't contain the invalid element
-    let currentElement = aInvalidElements.queryElementAt(0, Ci.nsISupports);
+    let currentElement = aInvalidElements[0];
     let focused = this.focusedElement;
     if (focused && focused.ownerGlobal.top !== currentElement.ownerGlobal.top) {
         return;
     }
 
     // Our focus listener will show the element's validation message
     currentElement.focus();
   },