--- a/dom/base/nsDOMWindowUtils.cpp
+++ b/dom/base/nsDOMWindowUtils.cpp
@@ -1665,17 +1665,17 @@ nsDOMWindowUtils::GetScrollbarSize(bool 
   *aWidth = nsPresContext::AppUnitsToIntCSSPixels(sizes.LeftRight());
   *aHeight = nsPresContext::AppUnitsToIntCSSPixels(sizes.TopBottom());
 
   return NS_OK;
 }
 
 NS_IMETHODIMP
 nsDOMWindowUtils::GetBoundsWithoutFlushing(nsIDOMElement *aElement,
-                                           nsISupports** aResult)
+                                           DOMRect** aResult)
 {
   nsCOMPtr<nsPIDOMWindowOuter> window = do_QueryReferent(mWindow);
   NS_ENSURE_STATE(window);
 
   nsresult rv;
   nsCOMPtr<nsIContent> content = do_QueryInterface(aElement, &rv);
   NS_ENSURE_SUCCESS(rv, rv);
 
@@ -1734,17 +1734,17 @@ nsDOMWindowUtils::FlushLayoutWithoutThro
     doc->FlushPendingNotifications(
       ChangesToFlush(FlushType::Layout, false /* flush animations */));
   }
 
   return NS_OK;
 }
 
 NS_IMETHODIMP
-nsDOMWindowUtils::GetRootBounds(nsISupports** aResult)
+nsDOMWindowUtils::GetRootBounds(DOMRect** aResult)
 {
   nsIDocument* doc = GetDocument();
   NS_ENSURE_STATE(doc);
 
   nsRect bounds(0, 0, 0, 0);
   nsIPresShell* presShell = doc->GetShell();
   if (presShell) {
     nsIScrollableFrame* sf = presShell->GetRootScrollFrameAsScrollable();

--- a/dom/base/nsFrameLoader.cpp
+++ b/dom/base/nsFrameLoader.cpp
@@ -2517,19 +2517,18 @@ GetContentParent(Element* aBrowser)
 {
   using ReturnTuple = Tuple<ContentParent*, TabParent*>;
 
   nsCOMPtr<nsIBrowser> browser = do_QueryInterface(aBrowser);
   if (!browser) {
     return ReturnTuple(nullptr, nullptr);
   }
 
-  nsCOMPtr<nsISupports> otherLoaderAsSupports;
-  browser->GetSameProcessAsFrameLoader(getter_AddRefs(otherLoaderAsSupports));
-  RefPtr<nsFrameLoader> otherLoader = do_QueryObject(otherLoaderAsSupports);
+  RefPtr<nsFrameLoader> otherLoader;
+  browser->GetSameProcessAsFrameLoader(getter_AddRefs(otherLoader));
   if (!otherLoader) {
     return ReturnTuple(nullptr, nullptr);
   }
 
   TabParent* tabParent = TabParent::GetFrom(otherLoader);
   if (tabParent &&
       tabParent->Manager() &&
       tabParent->Manager()->IsContentParent()) {

--- a/dom/base/nsIDroppedLinkHandler.idl
+++ b/dom/base/nsIDroppedLinkHandler.idl
@@ -2,16 +2,19 @@
  * 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 "nsISupports.idl"
 #include "nsIPrincipal.idl"
 
 interface nsIDOMEvent;
 
+webidl DragEvent;
+webidl DataTransfer;
+
 [scriptable, uuid(69E14F91-2E09-4CA6-A511-A715C99A2804)]
 interface nsIDroppedLinkItem : nsISupports
 {
   /**
    * Returns the URL of the link.
    */
   readonly attribute DOMString url;
 
@@ -32,20 +35,18 @@ interface nsIDroppedLinkHandler : nsISup
   /**
    * Determines if a link being dragged can be dropped and returns true if so.
    * aEvent should be a dragenter or dragover event.
    *
    * If aAllowSameDocument is false, drops are only allowed if the document
    * of the source of the drag is different from the destination. This check
    * includes any parent, sibling and child frames in the same content tree.
    * If true, the source is not checked.
-   *
-   * aEvent should be a DragEvent.  See bug 1444991.
    */
-  boolean canDropLink(in nsIDOMEvent aEvent, in boolean aAllowSameDocument);
+  boolean canDropLink(in DragEvent aEvent, in boolean aAllowSameDocument);
 
   /**
    * Given a drop event aEvent, determines the link being dragged and returns
    * it. If a uri is returned the caller can, for instance, load it. If null
    * is returned, there is no valid link to be dropped.
    *
    * A NS_ERROR_DOM_SECURITY_ERR error will be thrown and the event cancelled if
    * the receiving target should not load the uri for security reasons. This
@@ -53,67 +54,57 @@ interface nsIDroppedLinkHandler : nsISup
    *  - the source of the drag initiated a link for dragging that
    *    it itself cannot access. This prevents a source document from tricking
    *    the user into a dragging a chrome url, for example.
    *  - aDisallowInherit is true, and the URI being dropped would inherit the
    *    current document's security context (URI_INHERITS_SECURITY_CONTEXT).
    *
    * aName is filled in with the link title if it exists, or an empty string
    * otherwise.
-   *
-   * aEvent should be a DragEvent.  See bug 1444991.
    */
-  AString dropLink(in nsIDOMEvent aEvent, out AString aName,
+  AString dropLink(in DragEvent aEvent, out AString aName,
                    [optional] in boolean aDisallowInherit);
 
   /**
    * Given a drop event aEvent, determines links being dragged and returns
    * them. If links are returned the caller can, for instance, load them. If
    * the count of links is 0, there is no valid link to be dropped.
    *
    * A NS_ERROR_DOM_SECURITY_ERR error will be thrown and the event cancelled if
    * the receiving target should not load the uri for security reasons. This
    * will occur if any of the following conditions are true:
    *  - the source of the drag initiated a link for dragging that
    *    it itself cannot access. This prevents a source document from tricking
    *    the user into a dragging a chrome url, for example.
    *  - aDisallowInherit is true, and the URI being dropped would inherit the
    *    current document's security context (URI_INHERITS_SECURITY_CONTEXT).
-   *
-   * aEvent should be a DragEvent.  See bug 1444991.
    */
-  void dropLinks(in nsIDOMEvent aEvent,
+  void dropLinks(in DragEvent aEvent,
                  [optional] in boolean aDisallowInherit,
                  [optional] out unsigned long aCount,
                  [retval, array, size_is(aCount)] out nsIDroppedLinkItem aLinks);
 
   /**
    * Given a drop event aEvent, validate the extra URIs for the event,
    * this is used when the caller extracts yet another URIs from the dropped
    * text, like home button that splits the text with "|".
-   *
-   * aEvent should be a DragEvent.  See bug 1444991.
    */
-  void validateURIsForDrop(in nsIDOMEvent aEvent,
+  void validateURIsForDrop(in DragEvent aEvent,
                            in unsigned long aURIsCount,
                            [array, size_is(aURIsCount)] in wstring aURIs,
                            [optional] in boolean aDisallowInherit);
 
   /**
    * Given a dataTransfer, allows caller to determine and verify links being
    * dragged. Since drag/drop performs a roundtrip of parent, child, parent,
    * it allows the parent to verify that the child did not modify links
    * being dropped.
-   *
-   * @param dataTransfer is a DataTransfer.  See bug 1444991.
    */
-  void queryLinks(in nsISupports aDataTransfer,
+  void queryLinks(in DataTransfer aDataTransfer,
                   [optional] out unsigned long aCount,
                   [retval, array, size_is(aCount)] out nsIDroppedLinkItem aLinks);
 
   /**
    * Given a drop event aEvent, determines the triggering principal for the
    * event and returns it.
-   *
-   * aEvent should be a DragEvent.  See bug 1444991.
    */
-  nsIPrincipal getTriggeringPrincipal(in nsIDOMEvent aEvent);
+  nsIPrincipal getTriggeringPrincipal(in DragEvent aEvent);
 };

--- a/dom/browser-element/nsIBrowserElementAPI.idl
+++ b/dom/browser-element/nsIBrowserElementAPI.idl
@@ -3,16 +3,18 @@
 /* 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 "nsISupports.idl"
 
 interface nsIDOMDOMRequest;
 
+webidl FrameLoader;
+
 [scriptable, function, uuid(00d0e19d-bd67-491f-8e85-b9905224d3bb)]
 interface nsIBrowserElementNextPaintListener : nsISupports
 {
   void recvNextPaint();
 };
 
 %{C++
 #define BROWSER_ELEMENT_API_CONTRACTID "@mozilla.org/dom/browser-element-api;1"
@@ -34,18 +36,17 @@ interface nsIBrowserElementAPI : nsISupp
   const long FIND_FORWARD = 0;
   const long FIND_BACKWARD = 1;
 
   /**
    * Notify frame scripts that support the API to destroy.
    */
   void destroyFrameScripts();
 
-  // The argument should be a FrameLoader.  Fix that when bug 1444991 is fixed.
-  void setFrameLoader(in nsISupports frameLoader);
+  void setFrameLoader(in FrameLoader frameLoader);
 
   void sendMouseEvent(in DOMString type,
                       in uint32_t x,
                       in uint32_t y,
                       in uint32_t button,
                       in uint32_t clickCount,
                       in uint32_t mifiers);
   void sendTouchEvent(in DOMString aType,

--- a/dom/html/HTMLFormElement.cpp
+++ b/dom/html/HTMLFormElement.cpp
@@ -974,17 +974,17 @@ HTMLFormElement::NotifySubmitObservers(n
 
     bool loop = true;
     while (NS_SUCCEEDED(theEnum->HasMoreElements(&loop)) && loop) {
       theEnum->GetNext(getter_AddRefs(inst));
 
       nsCOMPtr<nsIFormSubmitObserver> formSubmitObserver(
                       do_QueryInterface(inst));
       if (formSubmitObserver) {
-        rv = formSubmitObserver->Notify(static_cast<nsIContent*>(this),
+        rv = formSubmitObserver->Notify(this,
                                         window ? window->GetCurrentInnerWindow() : nullptr,
                                         aActionURL,
                                         aCancelSubmit);
         NS_ENSURE_SUCCESS(rv, rv);
       }
       if (*aCancelSubmit) {
         return NS_OK;
       }
@@ -1972,17 +1972,17 @@ 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(static_cast<nsIContent*>(this),
+          observer->NotifyInvalidSubmit(this,
                                         static_cast<nsIArray*>(invalidElements));
         }
       }
 
       // The form is invalid. Observers have been alerted. Do not submit.
       return false;
     }
   } else {

--- a/dom/html/nsBrowserElement.cpp
+++ b/dom/html/nsBrowserElement.cpp
@@ -46,17 +46,17 @@ nsBrowserElement::InitBrowserElementAPI(
   }
 
   if (!mBrowserElementAPI) {
     mBrowserElementAPI = do_CreateInstance("@mozilla.org/dom/browser-element-api;1");
     if (NS_WARN_IF(!mBrowserElementAPI)) {
       return;
     }
   }
-  mBrowserElementAPI->SetFrameLoader(ToSupports(frameLoader));
+  mBrowserElementAPI->SetFrameLoader(frameLoader);
 }
 
 void
 nsBrowserElement::DestroyBrowserElementFrameScripts()
 {
   if (!mBrowserElementAPI) {
     return;
   }

--- a/dom/html/nsIFormSubmitObserver.idl
+++ b/dom/html/nsIFormSubmitObserver.idl
@@ -5,24 +5,24 @@
 
 
 #include "nsISupports.idl"
 
 interface mozIDOMWindow;
 interface nsIURI;
 interface nsIArray;
 
+webidl HTMLFormElement;
+
 [scriptable, uuid(867cb7e7-835d-408b-9788-d2834d284e03)]
 interface nsIFormSubmitObserver: nsISupports
 {
-  // formNode must be a HTMLFormElement (bug 1444991 can clean it up)
-  void notify(in nsISupports formNode, in mozIDOMWindow window, in nsIURI actionURL, out boolean cancelSubmit);
+  void notify(in HTMLFormElement formNode, in mozIDOMWindow window, in nsIURI actionURL, out boolean cancelSubmit);
 
-  // formNode must be a HTMLFormElement (bug 1444991 can clean it up)
-  void notifyInvalidSubmit(in nsISupports formNode,
+  void notifyInvalidSubmit(in HTMLFormElement formNode,
                            in nsIArray invalidElements);
 };
 
 %{C++
 #define NS_FORMSUBMIT_SUBJECT "formsubmit"
 #define NS_EARLYFORMSUBMIT_SUBJECT "earlyformsubmit"
 #define NS_FIRST_FORMSUBMIT_CATEGORY "firstformsubmit"
 #define NS_PASSWORDMANAGER_CATEGORY "passwordmanager"

--- a/dom/interfaces/base/nsIBrowser.idl
+++ b/dom/interfaces/base/nsIBrowser.idl
@@ -1,29 +1,28 @@
 /* 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 "nsISupports.idl"
 
 interface nsIPrincipal;
 
+webidl FrameLoader;
+
 [scriptable, uuid(14e5a0cb-e223-4202-95e8-fe53275193ea)]
 interface nsIBrowser : nsISupports
 {
   /**
    * Gets an optional frame loader that is "related" to this browser.
    * If this exists, then we should attempt to use the same content parent as
    * this frame loader for any new tab parents.  For example, view source
    * browsers set this to the frame loader for the original content to ensure
    * they are loaded in the same process as the content.
-   *
-   * This returns a FrameLoader, but we have no good way to represent
-   * one in xpidl.  Fix this when bug 1444991 is fixed.
    */
-  readonly attribute nsISupports sameProcessAsFrameLoader;
+  readonly attribute FrameLoader sameProcessAsFrameLoader;
 
   /*
    * Called by the child to inform the parent that links are dropped into
    * content area.
    *
    * @param linksCount length of links
    * @param links a flat array of url, name, and type for each link
    * @param triggeringPrincipal a principal that initiated loading

--- a/dom/interfaces/base/nsIDOMWindowUtils.idl
+++ b/dom/interfaces/base/nsIDOMWindowUtils.idl
@@ -44,16 +44,18 @@ interface nsIURI;
 interface nsIDOMEventTarget;
 interface nsIRunnable;
 interface nsITranslationNodeList;
 interface nsIJSRAIIHelper;
 interface nsIContentPermissionRequest;
 interface nsIObserver;
 interface nsIDOMStorage;
 
+webidl DOMRect;
+
 [scriptable, uuid(4d6732ca-9da7-4176-b8a1-8dde15cd0bf9)]
 interface nsIDOMWindowUtils : nsISupports {
 
   /**
    * Image animation mode of the window. When this attribute's value
    * is changed, the implementation should set all images in the window
    * to the given value. That is, when set to kDontAnimMode, all images
    * will stop animating. The attribute's value must be one of the
@@ -837,19 +839,18 @@ interface nsIDOMWindowUtils : nsISupport
    * Returns the scrollbar width of the window's scroll frame.
    *
    * @param aFlushLayout flushes layout if true. Otherwise, no flush occurs.
    */
   void getScrollbarSize(in boolean aFlushLayout, out long aWidth, out long aHeight);
 
   /**
    * Returns the given element's bounds without flushing pending layout changes.
-   * The returned object is a DOMRect (bug 1444991 may remove this walkaround).
    */
-  nsISupports getBoundsWithoutFlushing(in nsIDOMElement aElement);
+  DOMRect getBoundsWithoutFlushing(in nsIDOMElement aElement);
 
   const long FLUSH_NONE = -1;
   const long FLUSH_STYLE = 0;
   const long FLUSH_LAYOUT = 1;
   const long FLUSH_DISPLAY = 2;
 
   /**
    * Returns true if a flush of the given type is needed.
@@ -861,19 +862,18 @@ interface nsIDOMWindowUtils : nsISupport
    * animations.
    */
   void flushLayoutWithoutThrottledAnimations();
 
   /**
    * Returns the bounds of the window's currently loaded document. This will
    * generally be (0, 0, pageWidth, pageHeight) but in some cases (e.g. RTL
    * documents) may have a negative left value.
-   * The returned object is a DOMRect (bug 1444991 may remove this walkaround).
    */
-  nsISupports getRootBounds();
+  DOMRect getRootBounds();
 
   /**
    * Get IME open state. TRUE means 'Open', otherwise, 'Close'.
    * This property works only when IMEEnabled is IME_STATUS_ENABLED.
    */
   readonly attribute boolean IMEIsOpen;
 
   /**

--- a/dom/ipc/ProcessHangMonitor.cpp
+++ b/dom/ipc/ProcessHangMonitor.cpp
@@ -179,17 +179,17 @@ public:
 
   NS_IMETHOD TerminateScript() override;
   NS_IMETHOD TerminateGlobal() override;
   NS_IMETHOD BeginStartingDebugger() override;
   NS_IMETHOD EndStartingDebugger() override;
   NS_IMETHOD TerminatePlugin() override;
   NS_IMETHOD UserCanceled() override;
 
-  NS_IMETHOD IsReportForBrowser(nsISupports* aFrameLoader, bool* aResult) override;
+  NS_IMETHOD IsReportForBrowser(nsFrameLoader* aFrameLoader, bool* aResult) override;
 
   // Called when a content process shuts down.
   void Clear() {
     mContentParent = nullptr;
     mActor = nullptr;
   }
 
   /**
@@ -1119,29 +1119,28 @@ HangMonitoredProcess::TerminatePlugin()
                            contentPid,
                            NS_LITERAL_CSTRING("HangMonitor"),
                            mDumpId,
                            Move(callback));
   return NS_OK;
 }
 
 NS_IMETHODIMP
-HangMonitoredProcess::IsReportForBrowser(nsISupports* aFrameLoader, bool* aResult)
+HangMonitoredProcess::IsReportForBrowser(nsFrameLoader* aFrameLoader, bool* aResult)
 {
   MOZ_RELEASE_ASSERT(NS_IsMainThread());
 
   if (!mActor) {
     *aResult = false;
     return NS_OK;
   }
 
-  RefPtr<nsFrameLoader> frameLoader = do_QueryObject(aFrameLoader);
-  NS_ENSURE_STATE(frameLoader);
+  NS_ENSURE_STATE(aFrameLoader);
 
-  TabParent* tp = TabParent::GetFrom(frameLoader);
+  TabParent* tp = TabParent::GetFrom(aFrameLoader);
   if (!tp) {
     *aResult = false;
     return NS_OK;
   }
 
   *aResult = mContentParent == tp->Manager();
   return NS_OK;
 }

--- a/dom/ipc/nsIHangReport.idl
+++ b/dom/ipc/nsIHangReport.idl
@@ -3,16 +3,18 @@
 /* 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 "nsISupports.idl"
 
 interface nsIDOMElement;
 
+webidl FrameLoader;
+
 /**
  * When a content process hangs, Gecko notifies "process-hang-report" observers
  * and passes an nsIHangReport for the subject parameter. There is at most one
  * nsIHangReport associated with a given content process. As long as the content
  * process stays stuck, the "process-hang-report" observer will continue to be
  * notified at regular intervals (approximately once per second). The content
  * process will continue to run uninhibitedly during this time.
  */
@@ -60,11 +62,11 @@ interface nsIHangReport : nsISupports
 
   // Inform the content process that the slow script debugger has finished
   // spinning up. The content process will run a nested event loop until this
   // method is called.
   // Only valid for SLOW_SCRIPT reports.
   void endStartingDebugger();
 
   // Inquire whether the report is for a content process loaded by the given
-  // frameloader.  Make this take a FrameLoader once bug 1444991 is fixed.
-  bool isReportForBrowser(in nsISupports aFrameLoader);
+  // frameloader.
+  bool isReportForBrowser(in FrameLoader aFrameLoader);
 };

--- a/editor/nsIEditActionListener.idl
+++ b/editor/nsIEditActionListener.idl
@@ -3,16 +3,17 @@
  * 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 "nsISupports.idl"
 #include "domstubs.idl"
 
 interface nsISelection;
 
+webidl CharacterData;
 
 /*
 Editor Action Listener interface to outside world
 */
 
 
 /**
  * A generic editor action listener interface.
@@ -74,47 +75,47 @@ interface nsIEditActionListener : nsISup
    */
   void DidJoinNodes(in nsIDOMNode aLeftNode,
                           in nsIDOMNode aRightNode,
                           in nsIDOMNode aParent,
                           in nsresult    aResult);
 
   /**
    * Called after the editor inserts text.
-   * @param aTextNode   This node getting inserted text.  Should be a CharacterData after bug 1444991.
+   * @param aTextNode   This node getting inserted text.
    * @param aOffset     The offset in aTextNode to insert at.
    * @param aString     The string that gets inserted.
    * @param aResult     The result of the insert text operation.
    */
-  void DidInsertText(in nsISupports aTextNode,
+  void DidInsertText(in CharacterData       aTextNode,
                      in long                aOffset,
                      in DOMString           aString,
                      in nsresult            aResult);
 
   /**
    * Called before the editor deletes text.
-   * @param aTextNode   This node getting text deleted.  Should be a CharacterData after bug 1444991.
+   * @param aTextNode   This node getting text deleted.
    * @param aOffset     The offset in aTextNode to delete at.
    * @param aLength     The amount of text to delete.
    */
-  void WillDeleteText(in nsISupports aTextNode,
+  void WillDeleteText(in CharacterData       aTextNode,
                       in long                aOffset,
                       in long                aLength);
 
   /**
    * Called before the editor deletes text.
-   * @param aTextNode   This node getting text deleted.  Should be a CharacterData after bug 1444991.
+   * @param aTextNode   This node getting text deleted.
    * @param aOffset     The offset in aTextNode to delete at.
    * @param aLength     The amount of text to delete.
    * @param aResult     The result of the delete text operation.
    */
-  void DidDeleteText(in nsISupports aTextNode,
+  void DidDeleteText(in CharacterData       aTextNode,
                      in long                aOffset,
                      in long                aLength,
-                     in nsresult              aResult);
+                     in nsresult            aResult);
 
   /**
    * Called before the editor deletes the selection.
    * @param aSelection   The selection to be deleted
    */
   void WillDeleteSelection(in nsISelection aSelection);
 
   /**

--- a/editor/spellchecker/TextServicesDocument.cpp
+++ b/editor/spellchecker/TextServicesDocument.cpp
@@ -3215,34 +3215,34 @@ NS_IMETHODIMP
 TextServicesDocument::DidCreateNode(const nsAString& aTag,
                                     nsIDOMNode* aNewNode,
                                     nsresult aResult)
 {
   return NS_OK;
 }
 
 NS_IMETHODIMP
-TextServicesDocument::DidInsertText(nsISupports* aTextNode,
+TextServicesDocument::DidInsertText(CharacterData* aTextNode,
                                     int32_t aOffset,
                                     const nsAString& aString,
                                     nsresult aResult)
 {
   return NS_OK;
 }
 
 NS_IMETHODIMP
-TextServicesDocument::WillDeleteText(nsISupports* aTextNode,
+TextServicesDocument::WillDeleteText(CharacterData* aTextNode,
                                      int32_t aOffset,
                                      int32_t aLength)
 {
   return NS_OK;
 }
 
 NS_IMETHODIMP
-TextServicesDocument::DidDeleteText(nsISupports* aTextNode,
+TextServicesDocument::DidDeleteText(CharacterData* aTextNode,
                                     int32_t aOffset,
                                     int32_t aLength,
                                     nsresult aResult)
 {
   return NS_OK;
 }
 
 NS_IMETHODIMP

--- a/layout/xul/tree/nsITreeView.idl
+++ b/layout/xul/tree/nsITreeView.idl
@@ -4,16 +4,18 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "nsISupports.idl"
 
 interface nsITreeBoxObject;
 interface nsITreeSelection;
 interface nsITreeColumn;
 
+webidl DataTransfer;
+
 [scriptable, uuid(091116f0-0bdc-4b32-b9c8-c8d5a37cb088)]
 interface nsITreeView : nsISupports
 {
   /**
    * The total number of rows in the tree (including the offscreen rows).
    */
   readonly attribute long rowCount;
 
@@ -70,28 +72,24 @@ interface nsITreeView : nsISupports
   const short DROP_BEFORE = -1;
   const short DROP_ON = 0;
   const short DROP_AFTER = 1;
   /**
    * Methods used by the drag feedback code to determine if a drag is allowable at
    * the current location. To get the behavior where drops are only allowed on
    * items, such as the mailNews folder pane, always return false when
    * the orientation is not DROP_ON.
-   *
-   * @param dataTransfer should be a DataTransfer once bug 1444991 is fixed.
    */
-  boolean canDrop(in long index, in long orientation, in nsISupports dataTransfer);
+  boolean canDrop(in long index, in long orientation, in DataTransfer dataTransfer);
 
   /**
    * Called when the user drops something on this view. The |orientation| param
    * specifies before/on/after the given |row|.
-   *
-   * @param dataTransfer should be a DataTransfer once bug 1444991 is fixed.
    */
-  void drop(in long row, in long orientation, in nsISupports dataTransfer);
+  void drop(in long row, in long orientation, in DataTransfer dataTransfer);
 
   /**
    * Methods used by the tree to draw thread lines in the tree.
    * getParentIndex is used to obtain the index of a parent row.
    * If there is no parent row, getParentIndex returns -1.
    */
   long getParentIndex(in long rowIndex);
 

--- a/layout/xul/tree/nsTreeContentView.cpp
+++ b/layout/xul/tree/nsTreeContentView.cpp
@@ -363,17 +363,17 @@ bool
 nsTreeContentView::CanDrop(int32_t aRow, int32_t aOrientation,
                            DataTransfer* aDataTransfer, ErrorResult& aError)
 {
   return CanDrop(aRow, aOrientation, aError);
 }
 
 NS_IMETHODIMP
 nsTreeContentView::CanDrop(int32_t aIndex, int32_t aOrientation,
-                           nsISupports* aDataTransfer, bool *_retval)
+                           DataTransfer* aDataTransfer, bool *_retval)
 {
   ErrorResult rv;
   *_retval = CanDrop(aIndex, aOrientation, rv);
   return rv.StealNSResult();
 }
 
 void
 nsTreeContentView::Drop(int32_t aRow, int32_t aOrientation, ErrorResult& aError)
@@ -387,17 +387,17 @@ void
 nsTreeContentView::Drop(int32_t aRow, int32_t aOrientation,
                         DataTransfer* aDataTransfer, ErrorResult& aError)
 {
   Drop(aRow, aOrientation, aError);
 }
 
 NS_IMETHODIMP
 nsTreeContentView::Drop(int32_t aRow, int32_t aOrientation,
-                        nsISupports* aDataTransfer)
+                        DataTransfer* aDataTransfer)
 {
   ErrorResult rv;
   Drop(aRow, aOrientation, rv);
   return rv.StealNSResult();
 }
 
 int32_t
 nsTreeContentView::GetParentIndex(int32_t aRow, ErrorResult& aError)

--- a/security/manager/pki/nsASN1Tree.cpp
+++ b/security/manager/pki/nsASN1Tree.cpp
@@ -393,25 +393,25 @@ nsNSSASN1Tree::PerformActionOnRow(const 
 
 NS_IMETHODIMP
 nsNSSASN1Tree::PerformActionOnCell(const char16_t*, int32_t, nsITreeColumn*)
 {
   return NS_OK;
 }
 
 NS_IMETHODIMP
-nsNSSASN1Tree::CanDrop(int32_t, int32_t, nsISupports*, bool* _retval)
+nsNSSASN1Tree::CanDrop(int32_t, int32_t, mozilla::dom::DataTransfer*, bool* _retval)
 {
   NS_ENSURE_ARG_POINTER(_retval);
   *_retval = false;
   return NS_OK;
 }
 
 NS_IMETHODIMP
-nsNSSASN1Tree::Drop(int32_t, int32_t, nsISupports*)
+nsNSSASN1Tree::Drop(int32_t, int32_t, mozilla::dom::DataTransfer*)
 {
   return NS_OK;
 }
 
 NS_IMETHODIMP
 nsNSSASN1Tree::IsSorted(bool* _retval)
 {
   NS_ENSURE_ARG_POINTER(_retval);

--- a/security/manager/ssl/nsCertTree.cpp
+++ b/security/manager/ssl/nsCertTree.cpp
@@ -1219,30 +1219,31 @@ nsCertTree::dumpMap()
   }
 }
 #endif
 
 //
 // CanDrop
 //
 NS_IMETHODIMP nsCertTree::CanDrop(int32_t index, int32_t orientation,
-                                  nsISupports* aDataTransfer, bool *_retval)
+                                  mozilla::dom::DataTransfer* aDataTransfer,
+                                  bool *_retval)
 {
   NS_ENSURE_ARG_POINTER(_retval);
   *_retval = false;
 
   return NS_OK;
 }
 
 
 //
 // Drop
 //
 NS_IMETHODIMP nsCertTree::Drop(int32_t row, int32_t orient,
-			       nsISupports* aDataTransfer)
+			                         mozilla::dom::DataTransfer* aDataTransfer)
 {
   return NS_OK;
 }
 
 
 //
 // IsSorted
 //

--- a/widget/android/fennec/ThumbnailHelper.h
+++ b/widget/android/fennec/ThumbnailHelper.h
@@ -235,26 +235,24 @@ public:
         if (!window || !aData) {
             java::ThumbnailHelper::NotifyThumbnail(
                     aData, aTab, /* success */ false, /* store */ false);
             return;
         }
 
         // take a screenshot, as wide as possible, proportional to the destination size
         nsCOMPtr<nsIDOMWindowUtils> utils = do_GetInterface(window);
-        nsCOMPtr<nsISupports> rectSupports;
+        RefPtr<DOMRect> rect;
         if (!utils ||
-                NS_FAILED(utils->GetRootBounds(getter_AddRefs(rectSupports))) ||
-                !rectSupports) {
+                NS_FAILED(utils->GetRootBounds(getter_AddRefs(rect))) ||
+                !rect) {
             java::ThumbnailHelper::NotifyThumbnail(
                     aData, aTab, /* success */ false, /* store */ false);
             return;
         }
-        // this is safe, as GetRootBounds returns a DOMRect instance.
-        DOMRect* rect = DOMRect::FromSupports(rectSupports);
         float pageLeft = rect->Left();
         float pageTop = rect->Top();
         float pageWidth = rect->Width();
         float pageHeight = rect->Height();
         if (int32_t(pageWidth) == 0 || int32_t(pageHeight) == 0) {
             java::ThumbnailHelper::NotifyThumbnail(
                     aData, aTab, /* success */ false, /* store */ false);
             return;

--- a/widget/nsBaseDragService.cpp
+++ b/widget/nsBaseDragService.cpp
@@ -199,29 +199,28 @@ nsBaseDragService::GetData(nsITransferab
 NS_IMETHODIMP
 nsBaseDragService::IsDataFlavorSupported(const char *aDataFlavor,
                                          bool *_retval)
 {
   return NS_ERROR_FAILURE;
 }
 
 NS_IMETHODIMP
-nsBaseDragService::GetDataTransferXPCOM(nsISupports** aDataTransfer)
+nsBaseDragService::GetDataTransferXPCOM(DataTransfer** aDataTransfer)
 {
   *aDataTransfer = mDataTransfer;
   NS_IF_ADDREF(*aDataTransfer);
   return NS_OK;
 }
 
 NS_IMETHODIMP
-nsBaseDragService::SetDataTransferXPCOM(nsISupports* aDataTransfer)
+nsBaseDragService::SetDataTransferXPCOM(DataTransfer* aDataTransfer)
 {
-  RefPtr<DataTransfer> dataTransfer = do_QueryObject(aDataTransfer);
-  NS_ENSURE_STATE(dataTransfer);
-  mDataTransfer = dataTransfer.forget();
+  NS_ENSURE_STATE(aDataTransfer);
+  mDataTransfer = aDataTransfer;
   return NS_OK;
 }
 
 DataTransfer*
 nsBaseDragService::GetDataTransfer()
 {
   return mDataTransfer;
 }
@@ -276,38 +275,33 @@ nsBaseDragService::InvokeDragSession(nsI
 NS_IMETHODIMP
 nsBaseDragService::InvokeDragSessionWithImage(nsIDOMNode* aDOMNode,
                                               const nsACString& aPrincipalURISpec,
                                               nsIArray* aTransferableArray,
                                               nsIScriptableRegion* aRegion,
                                               uint32_t aActionType,
                                               nsIDOMNode* aImage,
                                               int32_t aImageX, int32_t aImageY,
-                                              nsIDOMEvent* aDragEvent,
+                                              DragEvent* aDragEvent,
                                               DataTransfer* aDataTransfer)
 {
   NS_ENSURE_TRUE(aDragEvent, NS_ERROR_NULL_POINTER);
   NS_ENSURE_TRUE(aDataTransfer, NS_ERROR_NULL_POINTER);
   NS_ENSURE_TRUE(mSuppressLevel == 0, NS_ERROR_FAILURE);
 
   mDataTransfer = aDataTransfer;
   mSelection = nullptr;
   mHasImage = true;
   mDragPopup = nullptr;
   mImage = aImage;
   mImageOffset = CSSIntPoint(aImageX, aImageY);
 
-  DragEvent* dragEvent = aDragEvent->InternalDOMEvent()->AsDragEvent();
-  if (NS_WARN_IF(!dragEvent)) {
-    return NS_ERROR_INVALID_ARG;
-  }
-
-  mScreenPosition.x = dragEvent->ScreenX(CallerType::System);
-  mScreenPosition.y = dragEvent->ScreenY(CallerType::System);
-  mInputSource = dragEvent->MozInputSource();
+  mScreenPosition.x = aDragEvent->ScreenX(CallerType::System);
+  mScreenPosition.y = aDragEvent->ScreenY(CallerType::System);
+  mInputSource = aDragEvent->MozInputSource();
 
   nsresult rv = InvokeDragSession(aDOMNode, aPrincipalURISpec,
                                   aTransferableArray,
                                   aRegion, aActionType,
                                   nsIContentPolicy::TYPE_INTERNAL_IMAGE);
 
   if (NS_FAILED(rv)) {
     mImage = nullptr;
@@ -318,38 +312,33 @@ nsBaseDragService::InvokeDragSessionWith
   return rv;
 }
 
 NS_IMETHODIMP
 nsBaseDragService::InvokeDragSessionWithSelection(nsISelection* aSelection,
                                                   const nsACString& aPrincipalURISpec,
                                                   nsIArray* aTransferableArray,
                                                   uint32_t aActionType,
-                                                  nsIDOMEvent* aDragEvent,
+                                                  DragEvent* aDragEvent,
                                                   DataTransfer* aDataTransfer)
 {
   NS_ENSURE_TRUE(aSelection, NS_ERROR_NULL_POINTER);
   NS_ENSURE_TRUE(aDragEvent, NS_ERROR_NULL_POINTER);
   NS_ENSURE_TRUE(mSuppressLevel == 0, NS_ERROR_FAILURE);
 
   mDataTransfer = aDataTransfer;
   mSelection = aSelection ? aSelection->AsSelection() : nullptr;
   mHasImage = true;
   mDragPopup = nullptr;
   mImage = nullptr;
   mImageOffset = CSSIntPoint();
 
-  DragEvent* dragEvent = aDragEvent->InternalDOMEvent()->AsDragEvent();
-  if (NS_WARN_IF(!dragEvent)) {
-    return NS_ERROR_INVALID_ARG;
-  }
-
-  mScreenPosition.x = dragEvent->ScreenX(CallerType::System);
-  mScreenPosition.y = dragEvent->ScreenY(CallerType::System);
-  mInputSource = dragEvent->MozInputSource();
+  mScreenPosition.x = aDragEvent->ScreenX(CallerType::System);
+  mScreenPosition.y = aDragEvent->ScreenY(CallerType::System);
+  mInputSource = aDragEvent->MozInputSource();
 
   // just get the focused node from the selection
   // XXXndeakin this should actually be the deepest node that contains both
   // endpoints of the selection
   nsCOMPtr<nsIDOMNode> node;
   aSelection->GetFocusNode(getter_AddRefs(node));
 
   nsresult rv = InvokeDragSession(node, aPrincipalURISpec,

--- a/widget/nsIDragService.idl
+++ b/widget/nsIDragService.idl
@@ -9,16 +9,18 @@
 #include "nsIDragSession.idl"
 #include "nsIScriptableRegion.idl"
 #include "nsIContentPolicy.idl"
 
 interface nsIDOMNode;
 interface nsIDOMEvent;
 interface nsISelection;
 
+webidl DragEvent;
+
 %{C++
 #include "mozilla/EventForwards.h"
 
 namespace mozilla {
 namespace dom {
 class ContentParent;
 class DataTransfer;
 } // namespace dom
@@ -81,45 +83,41 @@ interface nsIDragService : nsISupports
    * <canvas>, the drag image is taken from the image data. If the element is in
    * a document, it will be rendered at its displayed size, othewise, it will be
    * rendered at its real size. For other types of elements, the element is
    * rendered into an offscreen buffer in the same manner as it is currently
    * displayed. The document selection is hidden while drawing.
    *
    * The aDragEvent must be supplied as the current screen coordinates of the
    * event are needed to calculate the image location.
-   *
-   * aDragEvent should be a DragEvent.  See bug 1444991.
    */
   [noscript]
   void invokeDragSessionWithImage(in nsIDOMNode aDOMNode,
                                   in AUTF8String aPrincipalURISpec,
                                   in nsIArray aTransferableArray,
                                   in nsIScriptableRegion aRegion,
                                   in unsigned long aActionType,
                                   in nsIDOMNode aImage,
                                   in long aImageX,
                                   in long aImageY,
-                                  in nsIDOMEvent aDragEvent,
+                                  in DragEvent aDragEvent,
                                   in DataTransferPtr aDataTransfer);
 
   /**
    * Start a modal drag session using the selection as the drag image.
    * The aDragEvent must be supplied as the current screen coordinates of the
    * event are needed to calculate the image location.
    *
    * Note: This method is deprecated for non-native code.
-   *
-   * aDragEvent should be a DragEvent.  See bug 1444991.
    */
   void invokeDragSessionWithSelection(in nsISelection aSelection,
                                       in AUTF8String aPrincipalURISpec,
                                       in nsIArray aTransferableArray,
                                       in unsigned long aActionType,
-                                      in nsIDOMEvent aDragEvent,
+                                      in DragEvent aDragEvent,
                                       in DataTransferPtr aDataTransfer);
 
   /**
     * Returns the current Drag Session
     */
   nsIDragSession getCurrentSession();
 
   /**

--- a/widget/nsIDragSession.idl
+++ b/widget/nsIDragSession.idl
@@ -5,30 +5,25 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "nsISupports.idl"
 #include "nsITransferable.idl"
 
 
 %{ C++
 #include "nsSize.h"
-
-namespace mozilla {
-namespace dom {
-class DataTransfer;
-} // namespace dom
-} // namespace mozilla
 %}
 
 native nsSize (nsSize);
-[ptr] native DataTransferPtr(mozilla::dom::DataTransfer);
 
 interface nsIDOMDocument;
 interface nsIDOMNode;
 
+webidl DataTransfer;
+
 [scriptable, builtinclass, uuid(25bce737-73f0-43c7-bc20-c71044a73c5a)]
 interface nsIDragSession : nsISupports
 {
   /**
     * Set the current state of the drag, whether it can be dropped or not.
     * usually the target "frame" sets this so the native system can render the correct feedback
     */
   attribute boolean canDrop;
@@ -70,23 +65,22 @@ interface nsIDragSession : nsISupports
   /**
    * The URI spec of the triggering principal.  This may be different than
    * sourceNode's principal when sourceNode is xul:browser and the drag is
    * triggered in a browsing context inside it.
    */
   attribute AUTF8String triggeringPrincipalURISpec;
 
   /**
-   * The data transfer object for the current drag.  Should become a
-   * DataTransfer once bug 1444991 is fixed.
+   * The data transfer object for the current drag.
    */
   [binaryname(DataTransferXPCOM)]
-  attribute nsISupports dataTransfer;
-  [notxpcom, nostdcall] DataTransferPtr getDataTransfer();
-  [notxpcom, nostdcall] void setDataTransfer(in DataTransferPtr aDataTransfer);
+  attribute DataTransfer dataTransfer;
+  [notxpcom, nostdcall] DataTransfer getDataTransfer();
+  [notxpcom, nostdcall] void setDataTransfer(in DataTransfer aDataTransfer);
 
   /**
     * Get data from a Drag&Drop. Can be called while the drag is in process
     * or after the drop has completed.
     *
     * @param  aTransferable the transferable for the data to be put into
     * @param  aItemIndex which of multiple drag items, zero-based
     */