--- a/dom/html/HTMLSelectElement.cpp
+++ b/dom/html/HTMLSelectElement.cpp
@@ -1880,16 +1880,38 @@ HTMLSelectElement::SetSelectionChanged(b
 void
 HTMLSelectElement::UpdateSelectedOptions()
 {
   if (mSelectedOptions) {
     mSelectedOptions->SetDirty();
   }
 }
 
+bool
+HTMLSelectElement::OpenInParentProcess()
+{
+  nsIFormControlFrame* formControlFrame = GetFormControlFrame(false);
+  nsIComboboxControlFrame* comboFrame = do_QueryFrame(formControlFrame);
+  if (comboFrame) {
+    return comboFrame->IsOpenInParentProcess();
+  }
+
+  return false;
+}
+
+void
+HTMLSelectElement::SetOpenInParentProcess(bool aVal)
+{
+  nsIFormControlFrame* formControlFrame = GetFormControlFrame(false);
+  nsIComboboxControlFrame* comboFrame = do_QueryFrame(formControlFrame);
+  if (comboFrame) {
+    comboFrame->SetOpenInParentProcess(aVal);
+  }
+}
+
 JSObject*
 HTMLSelectElement::WrapNode(JSContext* aCx, JS::Handle<JSObject*> aGivenProto)
 {
   return HTMLSelectElementBinding::Wrap(aCx, this, aGivenProto);
 }
 
 } // namespace dom
 } // namespace mozilla

--- a/dom/html/HTMLSelectElement.h
+++ b/dom/html/HTMLSelectElement.h
@@ -426,16 +426,19 @@ public:
   /**
    * Is this a combobox?
    */
   bool IsCombobox() const
   {
     return !Multiple() && Size() <= 1;
   }
 
+  bool OpenInParentProcess();
+  void SetOpenInParentProcess(bool aVal);
+
 protected:
   virtual ~HTMLSelectElement();
 
   friend class SafeOptionListMutation;
 
   // Helper Methods
   /**
    * Check whether the option specified by the index is selected

--- a/dom/webidl/HTMLSelectElement.webidl
+++ b/dom/webidl/HTMLSelectElement.webidl
@@ -53,8 +53,13 @@ interface HTMLSelectElement : HTMLElemen
   boolean reportValidity();
   void setCustomValidity(DOMString error);
 
 // NYI:  readonly attribute NodeList labels;
 
   // https://www.w3.org/Bugs/Public/show_bug.cgi?id=20720
   void remove();
 };
+
+partial interface HTMLSelectElement {
+  [ChromeOnly]
+  attribute boolean openInParentProcess;
+};

--- a/layout/forms/nsComboboxControlFrame.cpp
+++ b/layout/forms/nsComboboxControlFrame.cpp
@@ -231,16 +231,17 @@ nsComboboxControlFrame::nsComboboxContro
   , mDisplayISize(0)
   , mRecentSelectedIndex(NS_SKIP_NOTIFY_INDEX)
   , mDisplayedIndex(-1)
   , mLastDropDownBeforeScreenBCoord(nscoord_MIN)
   , mLastDropDownAfterScreenBCoord(nscoord_MIN)
   , mDroppedDown(false)
   , mInRedisplayText(false)
   , mDelayedShowDropDown(false)
+  , mIsOpenInParentProcess(false)
 {
   REFLOW_COUNTER_INIT()
 }
 
 //--------------------------------------------------------------
 nsComboboxControlFrame::~nsComboboxControlFrame()
 {
   REFLOW_COUNTER_DUMP("nsCCF");

--- a/layout/forms/nsComboboxControlFrame.h
+++ b/layout/forms/nsComboboxControlFrame.h
@@ -158,16 +158,27 @@ public:
   virtual int32_t GetIndexOfDisplayArea() override;
   /**
    * @note This method might destroy |this|.
    */
   NS_IMETHOD RedisplaySelectedText() override;
   virtual int32_t UpdateRecentIndex(int32_t aIndex) override;
   virtual void OnContentReset() override;
 
+
+  bool IsOpenInParentProcess() override
+  {
+    return mIsOpenInParentProcess;
+  }
+
+  void SetOpenInParentProcess(bool aVal) override
+  {
+    mIsOpenInParentProcess = aVal;
+  }
+
   // nsISelectControlFrame
   NS_IMETHOD AddOption(int32_t index) override;
   NS_IMETHOD RemoveOption(int32_t index) override;
   NS_IMETHOD DoneAddingChildren(bool aIsDone) override;
   NS_IMETHOD OnOptionSelected(int32_t aIndex, bool aSelected) override;
   NS_IMETHOD OnSetSelectedIndex(int32_t aOldIndex, int32_t aNewIndex) override;
 
   //nsIRollupListener
@@ -298,16 +309,18 @@ protected:
   nscoord               mLastDropDownAfterScreenBCoord;
   // Current state of the dropdown list, true is dropped down.
   bool                  mDroppedDown;
   // See comment in HandleRedisplayTextEvent().
   bool                  mInRedisplayText;
   // Acting on ShowDropDown(true) is delayed until we're focused.
   bool                  mDelayedShowDropDown;
 
+  bool                  mIsOpenInParentProcess;
+
   // static class data member for Bug 32920
   // only one control can be focused at a time
   static nsComboboxControlFrame* sFocused;
 
 #ifdef DO_REFLOW_COUNTER
   int32_t mReflowId;
 #endif
 };

--- a/layout/forms/nsIComboboxControlFrame.h
+++ b/layout/forms/nsIComboboxControlFrame.h
@@ -16,16 +16,19 @@ class nsIComboboxControlFrame : public n
 public:
   NS_DECL_QUERYFRAME_TARGET(nsIComboboxControlFrame)
 
   /**
    * Indicates whether the list is dropped down
    */
   virtual bool IsDroppedDown() = 0;
 
+  virtual bool IsOpenInParentProcess() = 0;
+  virtual void SetOpenInParentProcess(bool aVal) = 0;
+
   /**
    * Shows or hides the drop down
    */
   virtual void ShowDropDown(bool aDoDropDown) = 0;
 
   /**
    * Gets the Drop Down List
    */

--- a/toolkit/modules/SelectContentHelper.jsm
+++ b/toolkit/modules/SelectContentHelper.jsm
@@ -58,31 +58,33 @@ this.SelectContentHelper.prototype = {
       // we'll poke a DeferredTask to update the parent sometime
       // in the very near future.
       this._updateTimer.arm();
     });
     this.mut.observe(this.element, {childList: true, subtree: true});
   },
 
   uninit: function() {
+    this.element.openInParentProcess = false;
     this.global.removeMessageListener("Forms:SelectDropDownItem", this);
     this.global.removeMessageListener("Forms:DismissedDropDown", this);
     this.global.removeMessageListener("Forms:MouseOver", this);
     this.global.removeMessageListener("Forms:MouseOut", this);
     this.global.removeEventListener("pagehide", this);
     this.global.removeEventListener("mozhidedropdown", this);
     this.element = null;
     this.global = null;
     this.mut.disconnect();
     this._updateTimer.disarm();
     this._updateTimer = null;
     gOpen = false;
   },
 
   showDropDown: function() {
+    this.element.openInParentProcess = true;
     let rect = this._getBoundingContentRect();
     this.global.sendAsyncMessage("Forms:ShowDropDown", {
       rect: rect,
       options: this._buildOptionList(),
       selectedIndex: this.element.selectedIndex,
       direction: getComputedDirection(this.element)
     });
     gOpen = true;