diff --git a/dom/base/Navigator.cpp b/dom/base/Navigator.cpp
index 0f7f2b756e..026ae15836 100644
--- a/dom/base/Navigator.cpp
+++ b/dom/base/Navigator.cpp
@@ -1532,6 +1532,14 @@ Navigator::Clipboard()
   return mClipboard;
 }
 
+/* static */
+bool
+Navigator::Webdriver()
+{
+  // We don't support Selenium or marionette, so this is always false.
+  return false;
+}
+
 #ifdef MOZ_EME
 static nsCString
 ToCString(const nsString& aString)
diff --git a/dom/base/Navigator.h b/dom/base/Navigator.h
index 10f9573ee9..d674f5cedc 100644
--- a/dom/base/Navigator.h
+++ b/dom/base/Navigator.h
@@ -219,6 +219,8 @@ public:
   
   dom::Clipboard* Clipboard();
 
+  static bool Webdriver();
+
   void GetLanguages(nsTArray<nsString>& aLanguages);
 
   bool MozE10sEnabled();
diff --git a/dom/html/HTMLButtonElement.cpp b/dom/html/HTMLButtonElement.cpp
index e3fdbb4d32..7f0852d11d 100644
--- a/dom/html/HTMLButtonElement.cpp
+++ b/dom/html/HTMLButtonElement.cpp
@@ -33,6 +33,7 @@
 #include "nsFocusManager.h"
 #include "mozilla/dom/HTMLFormElement.h"
 #include "mozAutoDocUpdate.h"
+#include "mozilla/Preferences.h"
 
 #define NS_IN_SUBMIT_CLICK      (1 << 0)
 #define NS_OUTER_ACTIVATE_EVENT (1 << 1)
@@ -186,6 +187,9 @@ HTMLButtonElement::ParseAttribute(int32_t aNamespaceID,
     }
 
     if (aAttribute == nsGkAtoms::formmethod) {
+      if (Preferences::GetBool("dom.dialog_element.enabled", true)) {
+        return aResult.ParseEnumValue(aValue, kFormMethodTableDialogEnabled, false);
+      }
       return aResult.ParseEnumValue(aValue, kFormMethodTable, false);
     }
     if (aAttribute == nsGkAtoms::formenctype) {
diff --git a/dom/html/HTMLFormElement.cpp b/dom/html/HTMLFormElement.cpp
index c8bb526379..d2b39461f1 100644
--- a/dom/html/HTMLFormElement.cpp
+++ b/dom/html/HTMLFormElement.cpp
@@ -321,6 +321,9 @@ HTMLFormElement::ParseAttribute(int32_t aNamespaceID,
 {
   if (aNamespaceID == kNameSpaceID_None) {
     if (aAttribute == nsGkAtoms::method) {
+      if (Preferences::GetBool("dom.dialog_element.enabled", true)) {
+        return aResult.ParseEnumValue(aValue, kFormMethodTableDialogEnabled, false);
+      }
       return aResult.ParseEnumValue(aValue, kFormMethodTable, false);
     }
     if (aAttribute == nsGkAtoms::enctype) {
@@ -673,6 +676,30 @@ HTMLFormElement::DoSubmit(WidgetEvent* aEvent)
 
   mSubmitInitiatedFromUserInput = EventStateManager::IsHandlingUserInput();
 
+  //
+  // perform the submission
+  //
+  if (!submission) {
+    mIsSubmitting = false;
+#ifdef DEBUG
+    HTMLDialogElement* dialog = nullptr;
+    for (nsIContent* parent = GetParent(); parent;
+         parent = parent->GetParent()) {
+      dialog = HTMLDialogElement::FromContentOrNull(parent);
+      if (dialog) {
+        break;
+      }
+    }
+    MOZ_ASSERT(!dialog || !dialog->Open());
+#endif
+    return NS_OK;
+  }
+
+  if (DialogFormSubmission* dialogSubmission =
+          submission->GetAsDialogSubmission()) {
+    return SubmitDialog(dialogSubmission);
+  }
+
   if(mDeferSubmission) {
     // we are in an event handler, JS submitted so we have to
     // defer this submission. let's remember it and return
@@ -683,9 +710,6 @@ HTMLFormElement::DoSubmit(WidgetEvent* aEvent)
     return NS_OK;
   }
 
-  //
-  // perform the submission
-  //
   return SubmitSubmission(submission);
 }
 
@@ -722,8 +746,10 @@ HTMLFormElement::BuildSubmission(HTMLFormSubmission** aFormSubmission,
   //
   // Dump the data into the submission object
   //
-  rv = WalkFormElements(*aFormSubmission);
-  NS_ENSURE_SUBMIT_SUCCESS(rv);
+  if (!(*aFormSubmission)->GetAsDialogSubmission()) {
+    rv = WalkFormElements(*aFormSubmission);
+    NS_ENSURE_SUBMIT_SUCCESS(rv);
+  }
 
   return NS_OK;
 }
@@ -862,6 +888,22 @@ HTMLFormElement::SubmitSubmission(HTMLFormSubmission* aFormSubmission)
   return rv;
 }
 
+// https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#submit-dialog
+nsresult HTMLFormElement::SubmitDialog(DialogFormSubmission* aFormSubmission) {
+  mIsSubmitting = false;
+
+  // Close the dialog subject. If there is a result, let that be the return
+  // value.
+  HTMLDialogElement* dialog = aFormSubmission->DialogElement();
+  MOZ_ASSERT(dialog);
+
+  Optional<nsAString> retValue;
+  retValue = &aFormSubmission->ReturnValue();
+  dialog->Close(retValue);
+
+  return NS_OK;
+}
+
 nsresult
 HTMLFormElement::DoSecureToInsecureSubmitCheck(nsIURI* aActionURL,
                                                bool* aCancelSubmit)
diff --git a/dom/html/HTMLFormElement.h b/dom/html/HTMLFormElement.h
index f13eab9acc..102e24fdf9 100644
--- a/dom/html/HTMLFormElement.h
+++ b/dom/html/HTMLFormElement.h
@@ -483,6 +483,12 @@ protected:
    */
   nsresult SubmitSubmission(HTMLFormSubmission* aFormSubmission);
 
+  /**
+   * Submit a form[method=dialog]
+   * @param aFormSubmission the submission object
+   */
+  nsresult SubmitDialog(DialogFormSubmission* aFormSubmission);
+
   /**
    * Notify any submit observers of the submit.
    *
diff --git a/dom/html/HTMLFormSubmission.cpp b/dom/html/HTMLFormSubmission.cpp
index d447f51476..6a931858b3 100644
--- a/dom/html/HTMLFormSubmission.cpp
+++ b/dom/html/HTMLFormSubmission.cpp
@@ -944,6 +944,30 @@ HTMLFormSubmission::GetFromForm(nsGenericHTMLElement* aForm,
     charset.AssignLiteral("UTF-8");
   }
 
+  if (method == NS_FORM_METHOD_DIALOG) {
+    HTMLDialogElement* dialog = nullptr;
+    for (nsIContent* parent = aForm->GetParent(); parent;
+         parent = parent->GetParent()) {
+      dialog = HTMLDialogElement::FromContentOrNull(parent);
+      if (dialog) {
+        break;
+      }
+    }
+
+    // If there isn't one, or if it does not have an open attribute, do
+    // nothing.
+    if (!dialog || !dialog->Open()) {
+      return NS_ERROR_FAILURE;
+    }
+
+    nsAutoString result;
+    if (aOriginatingElement) {
+      aOriginatingElement->ResultForDialogSubmit(result);
+    }
+    *aFormSubmission = new DialogFormSubmission(result, charset, aOriginatingElement, dialog);
+    return NS_OK;
+  }
+
   // Choose encoder
   if (method == NS_FORM_METHOD_POST &&
       enctype == NS_FORM_ENCTYPE_MULTIPART) {
diff --git a/dom/html/HTMLFormSubmission.h b/dom/html/HTMLFormSubmission.h
index 506aa668df..d2ea8df8b5 100644
--- a/dom/html/HTMLFormSubmission.h
+++ b/dom/html/HTMLFormSubmission.h
@@ -7,6 +7,7 @@
 #define mozilla_dom_HTMLFormSubmission_h
 
 #include "mozilla/Attributes.h"
+#include "mozilla/dom/HTMLDialogElement.h"
 #include "nsCOMPtr.h"
 #include "nsIContent.h"
 #include "nsNCRFallbackEncoderWrapper.h"
@@ -121,6 +122,8 @@ public:
     return mOriginatingElement.get();
   }
 
+  virtual DialogFormSubmission* GetAsDialogSubmission() { return nullptr; }
+
 protected:
   /**
    * Can only be constructed by subclasses.
@@ -168,6 +171,50 @@ private:
   nsNCRFallbackEncoderWrapper mEncoder;
 };
 
+
+class DialogFormSubmission final : public HTMLFormSubmission {
+ public:
+  DialogFormSubmission(nsAString& aResult,
+                       const nsACString& aCharset, Element* aSubmitter,
+                       HTMLDialogElement* aDialogElement)
+      : HTMLFormSubmission(aCharset, aSubmitter)
+      , mDialogElement(aDialogElement)
+      , mReturnValue(aResult) {
+  }
+
+  nsresult AddNameValuePair(const nsAString& aName,
+                            const nsAString& aValue) override {
+    MOZ_CRASH("This method should not be called");
+    return NS_OK;
+  }
+
+  nsresult AddNameBlobOrNullPair(const nsAString& aName, Blob* aBlob) override {
+    MOZ_CRASH("This method should not be called");
+    return NS_OK;
+  }
+
+  nsresult AddNameDirectoryPair(const nsAString& aName,
+                                Directory* aDirectory) override {
+    MOZ_CRASH("This method should not be called");
+    return NS_OK;
+  }
+
+  nsresult GetEncodedSubmission(nsIURI* aURI, nsIInputStream** aPostDataStream) override {
+    MOZ_CRASH("This method should not be called");
+    return NS_OK;
+  }
+
+  DialogFormSubmission* GetAsDialogSubmission() override { return this; }
+
+  HTMLDialogElement* DialogElement() { return mDialogElement; }
+
+  nsString& ReturnValue() { return mReturnValue; }
+
+ private:
+  const RefPtr<HTMLDialogElement> mDialogElement;
+  nsString mReturnValue;
+};
+
 /**
  * Handle multipart/form-data encoding, which does files as well as normal
  * inputs.  This always does POST.
diff --git a/dom/html/HTMLFormSubmissionConstants.h b/dom/html/HTMLFormSubmissionConstants.h
index 261af61652..d3a880c58a 100644
--- a/dom/html/HTMLFormSubmissionConstants.h
+++ b/dom/html/HTMLFormSubmissionConstants.h
@@ -14,6 +14,13 @@ static const nsAttrValue::EnumTable kFormMethodTable[] = {
   { nullptr, 0 }
 };
 
+static const nsAttrValue::EnumTable kFormMethodTableDialogEnabled[] = {
+    {"get", NS_FORM_METHOD_GET},
+    {"post", NS_FORM_METHOD_POST},
+    {"dialog", NS_FORM_METHOD_DIALOG},
+    {nullptr, 0}
+};
+
 // Default method is 'get'.
 static const nsAttrValue::EnumTable* kFormDefaultMethod = &kFormMethodTable[0];
 
diff --git a/dom/html/HTMLInputElement.cpp b/dom/html/HTMLInputElement.cpp
index dd2d134f8e..7d9a4db1cd 100644
--- a/dom/html/HTMLInputElement.cpp
+++ b/dom/html/HTMLInputElement.cpp
@@ -1482,6 +1482,26 @@ NS_IMPL_STRING_ATTR(HTMLInputElement, Placeholder, placeholder)
 NS_IMPL_ENUM_ATTR_DEFAULT_VALUE(HTMLInputElement, Type, type,
                                 kInputDefaultType->tag)
 
+void HTMLInputElement::ResultForDialogSubmit(nsAString& aResult) {
+  if (mType == NS_FORM_INPUT_IMAGE) {
+    // Get a property set by the frame to find out where it was clicked.
+    nsIntPoint* lastClickedPoint =
+        static_cast<nsIntPoint*>(GetProperty(nsGkAtoms::imageClickedPoint));
+    int32_t x, y;
+    if (lastClickedPoint) {
+      x = lastClickedPoint->x;
+      y = lastClickedPoint->y;
+    } else {
+      x = y = 0;
+    }
+    aResult.AppendInt(x);
+    aResult.AppendLiteral(",");
+    aResult.AppendInt(y);
+  } else {
+    GetAttr(kNameSpaceID_None, nsGkAtoms::value, aResult);
+  }
+}
+
 NS_IMETHODIMP
 HTMLInputElement::GetAutocomplete(nsAString& aValue)
 {
@@ -5916,6 +5936,9 @@ HTMLInputElement::ParseAttribute(int32_t aNamespaceID,
       return aResult.ParseEnumValue(aValue, kFormMethodTable, false);
     }
     if (aAttribute == nsGkAtoms::formenctype) {
+      if (Preferences::GetBool("dom.dialog_element.enabled", true)) {
+        return aResult.ParseEnumValue(aValue, kFormMethodTableDialogEnabled, false);
+      }
       return aResult.ParseEnumValue(aValue, kFormEnctypeTable, false);
     }
     if (aAttribute == nsGkAtoms::autocomplete) {
diff --git a/dom/html/HTMLInputElement.h b/dom/html/HTMLInputElement.h
index 24fc7454ac..44e1f9c1ca 100644
--- a/dom/html/HTMLInputElement.h
+++ b/dom/html/HTMLInputElement.h
@@ -968,6 +968,8 @@ protected:
                                 const nsAttrValue* aOldValue,
                                 bool aNotify) override;
 
+  virtual void ResultForDialogSubmit(nsAString& aResult) override;
+  
   /**
    * Dispatch a select event. Returns true if the event was not cancelled.
    */
diff --git a/dom/html/nsGenericHTMLElement.h b/dom/html/nsGenericHTMLElement.h
index 56812b445b..d2b7f45e9c 100644
--- a/dom/html/nsGenericHTMLElement.h
+++ b/dom/html/nsGenericHTMLElement.h
@@ -861,6 +861,10 @@ public:
   static bool
   IsScrollGrabAllowed(JSContext*, JSObject*);
 
+  virtual inline void ResultForDialogSubmit(nsAString& aResult) {
+    GetAttr(kNameSpaceID_None, nsGkAtoms::value, aResult);
+  }
+
 protected:
   /**
    * Add/remove this element to the documents name cache
diff --git a/dom/html/nsIForm.h b/dom/html/nsIForm.h
index 44eedc5103..6c2b1d8623 100644
--- a/dom/html/nsIForm.h
+++ b/dom/html/nsIForm.h
@@ -12,6 +12,7 @@ class nsIFormControl;
 
 #define NS_FORM_METHOD_GET  0
 #define NS_FORM_METHOD_POST 1
+#define NS_FORM_METHOD_DIALOG 2
 #define NS_FORM_ENCTYPE_URLENCODED 0
 #define NS_FORM_ENCTYPE_MULTIPART  1
 #define NS_FORM_ENCTYPE_TEXTPLAIN  2
diff --git a/dom/html/nsIFormControl.h b/dom/html/nsIFormControl.h
index 067ac7cae8..1815c88543 100644
--- a/dom/html/nsIFormControl.h
+++ b/dom/html/nsIFormControl.h
@@ -16,6 +16,7 @@ namespace dom {
 class Element;
 class HTMLFieldSetElement;
 class HTMLFormSubmission;
+class DialogFormSubmission;
 } // namespace dom
 } // namespace mozilla
 
diff --git a/dom/webidl/Navigator.webidl b/dom/webidl/Navigator.webidl
index f2212de392..4541210a08 100644
--- a/dom/webidl/Navigator.webidl
+++ b/dom/webidl/Navigator.webidl
@@ -12,6 +12,7 @@
  * https://dvcs.w3.org/hg/gamepad/raw-file/default/gamepad.html#navigator-interface-extension
  * http://www.w3.org/TR/beacon/#sec-beacon-method
  * https://html.spec.whatwg.org/#navigatorconcurrenthardware
+ * https://w3c.github.io/webdriver/webdriver-spec.html#interface
  *
  * © Copyright 2004-2020 Apple Computer, Inc., Mozilla Foundation,
  * Opera Software ASA and Moonchild Productions. You are granted a license to use,
@@ -31,6 +32,7 @@ Navigator implements NavigatorStorageUtils;
 Navigator implements NavigatorConcurrentHardware;
 Navigator implements NavigatorStorage;
 Navigator implements NavigatorGlobalPrivacyControl;
+Navigator implements NavigatorAutomationInformation;
 
 [NoInterfaceObject, Exposed=(Window,Worker)]
 interface NavigatorID {
@@ -334,3 +336,10 @@ partial interface Navigator {
   [Pref="dom.events.asyncClipboard", SecureContext, SameObject]
   readonly attribute Clipboard clipboard;
 };
+
+// https://w3c.github.io/webdriver/webdriver-spec.html#interface
+[NoInterfaceObject, Exposed=Window]
+interface NavigatorAutomationInformation {
+  [Pref="dom.webdriver.enabled"]
+  readonly attribute boolean webdriver;
+};
diff --git a/gfx/qcms/transform-altivec.c b/gfx/qcms/transform-altivec.c
index 9dc17bc13f..df896db56c 100644
--- a/gfx/qcms/transform-altivec.c
+++ b/gfx/qcms/transform-altivec.c
@@ -29,7 +29,7 @@
 static const ALIGN float floatScaleX4 = FLOATSCALE;
 static const ALIGN float clampMaxValueX4 = CLAMPMAXVAL;
 
-inline vector float load_aligned_float(float *dataPtr)
+static inline vector float load_aligned_float(float *dataPtr)
 {
 	vector float data = vec_lde(0, dataPtr);
 	vector unsigned char moveToStart = vec_lvsl(0, dataPtr);
diff --git a/layout/forms/nsListControlFrame.cpp b/layout/forms/nsListControlFrame.cpp
index 4daa470a9e..0f6772a09a 100644
--- a/layout/forms/nsListControlFrame.cpp
+++ b/layout/forms/nsListControlFrame.cpp
@@ -2236,11 +2236,20 @@ nsListControlFrame::KeyDown(nsIDOMEvent* aKeyEvent)
         return NS_OK;
       }
 
+      // We don't want to preventDefault for escape key if the dropdown
+      // popup is not shown.
+      // Need to do the check before AboutToRollup because AboutToRollup
+      // may update the dropdown flags.
+      bool doPreventDefault =
+          !mComboboxFrame || mComboboxFrame->IsDroppedDownOrHasParentPopup();
+
       AboutToRollup();
-      // If the select element is a dropdown style, Enter key should be
+      // If the select element is a dropdown style, Escape key should be
       // consumed everytime since Escape key may be pressed accidentally after
       // the dropdown is closed by Escepe key.
-      aKeyEvent->PreventDefault();
+      if (doPreventDefault) {
+        aKeyEvent->PreventDefault();
+      }
       return NS_OK;
     }
     case NS_VK_PAGE_UP: {
diff --git a/layout/style/res/ua.css b/layout/style/res/ua.css
index ab51f67c53..bf32e66c56 100644
--- a/layout/style/res/ua.css
+++ b/layout/style/res/ua.css
@@ -320,6 +320,8 @@
   position: fixed;
   top: 0; left: 0;
   right: 0; bottom: 0;
+  /* This prevents undesired interactions with the selection code. */
+  -moz-user-select: none;
 }
 
 *|*:-moz-full-screen:not(:root)::backdrop {
diff --git a/modules/libpref/init/all.js b/modules/libpref/init/all.js
index 12d05de4a9..128130349a 100644
--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -5264,6 +5264,9 @@ pref("media.seekToNextFrame.enabled", true);
 // return the maximum number of cores that navigator.hardwareConcurrency returns
 pref("dom.maxHardwareConcurrency", 16);
 
+// Exposes the navigator.webdriver attribute.
+pref("dom.webdriver.enabled", true);
+
 // Shutdown the async osfile worker if it's no longer needed.
 pref("osfile.reset_worker_delay", 30000);