--- a/layout/printing/nsPrintData.cpp
+++ b/layout/printing/nsPrintData.cpp
@@ -20,17 +20,17 @@
 static mozilla::LazyLogModule gPrintingLog("printing");
 
 #define PR_PL(_p1)  MOZ_LOG(gPrintingLog, mozilla::LogLevel::Debug, _p1);
 
 //---------------------------------------------------
 //-- nsPrintData Class Impl
 //---------------------------------------------------
 nsPrintData::nsPrintData(ePrintDataType aType) :
-  mType(aType), mDebugFilePtr(nullptr), mPrintObject(nullptr), mSelectedPO(nullptr),
+  mType(aType), mDebugFilePtr(nullptr), mSelectedPO(nullptr),
   mPrintDocList(0), mIsIFrameSelected(false),
   mIsParentAFrameSet(false), mOnStartSent(false),
   mIsAborted(false), mPreparingForPrint(false), mDocWasToBeDestroyed(false),
   mShrinkToFit(false), mPrintFrameType(nsIPrintSettings::kFramesAsIs),
   mNumPrintablePages(0), mNumPagesPrinted(0),
   mShrinkRatio(1.0), mOrigDCScale(1.0), mPPEventListeners(nullptr),
   mBrandName(nullptr)
 {
@@ -79,18 +79,16 @@ nsPrintData::~nsPrintData()
         rv = mPrintDC->AbortDocument();
       }
       if (NS_FAILED(rv)) {
         // XXX nsPrintData::ShowPrintErrorDialog(rv);
       }
     }
   }
 
-  delete mPrintObject;
-
   if (mBrandName) {
     free(mBrandName);
   }
 }
 
 void nsPrintData::OnStartPrinting()
 {
   if (!mOnStartSent) {

--- a/layout/printing/nsPrintData.h
+++ b/layout/printing/nsPrintData.h
@@ -2,16 +2,17 @@
 /* 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/. */
 
 #ifndef nsPrintData_h___
 #define nsPrintData_h___
 
 #include "mozilla/Attributes.h"
+#include "mozilla/UniquePtr.h"
 
 // Interfaces
 #include "nsDeviceContext.h"
 #include "nsIPrintProgressParams.h"
 #include "nsIPrintSettings.h"
 #include "nsTArray.h"
 #include "nsCOMArray.h"
 
@@ -52,25 +53,29 @@ public:
 
   void DoOnStatusChange(nsresult aStatus);
 
 
   ePrintDataType               mType;            // the type of data this is (Printing or Print Preview)
   RefPtr<nsDeviceContext>   mPrintDC;
   FILE                        *mDebugFilePtr;    // a file where information can go to when printing
 
-  nsPrintObject *                mPrintObject;
-  nsPrintObject *                mSelectedPO;
+  mozilla::UniquePtr<nsPrintObject> mPrintObject;
+  nsPrintObject* mSelectedPO; // This is a non-owning pointer.
 
   nsCOMArray<nsIWebProgressListener> mPrintProgressListeners;
   nsCOMPtr<nsIPrintProgressParams> mPrintProgressParams;
 
   nsCOMPtr<nsPIDOMWindowOuter> mCurrentFocusWin; // cache a pointer to the currently focused window
 
+  // Array of non-owning pointers to all the nsPrintObjects owned by this
+  // nsPrintData. This includes this->mPrintObject, as well as all of its
+  // mKids (and their mKids, etc.)
   nsTArray<nsPrintObject*>    mPrintDocList;
+
   bool                        mIsIFrameSelected;
   bool                        mIsParentAFrameSet;
   bool                        mOnStartSent;
   bool                        mIsAborted;           // tells us the document is being aborted
   bool                        mPreparingForPrint;   // see comments above
   bool                        mDocWasToBeDestroyed; // see comments above
   bool                        mShrinkToFit;
   int16_t                     mPrintFrameType;

--- a/layout/printing/nsPrintEngine.cpp
+++ b/layout/printing/nsPrintEngine.cpp
@@ -161,18 +161,18 @@ static const char * gPrintRangeStr[]    
 
 #ifdef EXTENDED_DEBUG_PRINTING
 // Forward Declarations
 static void DumpPrintObjectsListStart(const char * aStr, nsTArray<nsPrintObject*> * aDocList);
 static void DumpPrintObjectsTree(nsPrintObject * aPO, int aLevel= 0, FILE* aFD = nullptr);
 static void DumpPrintObjectsTreeLayout(nsPrintObject * aPO,nsDeviceContext * aDC, int aLevel= 0, FILE * aFD = nullptr);
 
 #define DUMP_DOC_LIST(_title) DumpPrintObjectsListStart((_title), mPrt->mPrintDocList);
-#define DUMP_DOC_TREE DumpPrintObjectsTree(mPrt->mPrintObject);
-#define DUMP_DOC_TREELAYOUT DumpPrintObjectsTreeLayout(mPrt->mPrintObject, mPrt->mPrintDC);
+#define DUMP_DOC_TREE DumpPrintObjectsTree(mPrt->mPrintObject.get());
+#define DUMP_DOC_TREELAYOUT DumpPrintObjectsTreeLayout(mPrt->mPrintObject.get(), mPrt->mPrintDC);
 #else
 #define DUMP_DOC_LIST(_title)
 #define DUMP_DOC_TREE
 #define DUMP_DOC_TREELAYOUT
 #endif
 
 class nsScriptSuppressor
 {
@@ -352,17 +352,17 @@ nsPrintEngine::GetSeqFrameAndCountPagesI
 
   return NS_OK;
 }
 
 //-----------------------------------------------------------------
 nsresult nsPrintEngine::GetSeqFrameAndCountPages(nsIFrame*& aSeqFrame, int32_t& aCount)
 {
   NS_ASSERTION(mPrtPreview, "mPrtPreview can't be null!");
-  return GetSeqFrameAndCountPagesInternal(mPrtPreview->mPrintObject, aSeqFrame, aCount);
+  return GetSeqFrameAndCountPagesInternal(mPrtPreview->mPrintObject.get(), aSeqFrame, aCount);
 }
 //---------------------------------------------------------------------------------
 //-- Done: Methods needed by the DocViewer
 //---------------------------------------------------------------------------------
 
 
 //---------------------------------------------------------------------------------
 //-- Section: nsIWebBrowserPrint
@@ -508,44 +508,43 @@ nsPrintEngine::DoCommonPrint(bool       
       nsCOMPtr<nsIContentViewer> viewer;
       webContainer->GetContentViewer(getter_AddRefs(viewer));
       if (viewer && viewer->GetDocument() && viewer->GetDocument()->IsShowing()) {
         viewer->GetDocument()->OnPageHide(false, nullptr);
       }
     }
 
     nsAutoScriptBlocker scriptBlocker;
-    mPrt->mPrintObject = new nsPrintObject();
-    NS_ENSURE_TRUE(mPrt->mPrintObject, NS_ERROR_OUT_OF_MEMORY);
+    mPrt->mPrintObject = MakeUnique<nsPrintObject>();
     rv = mPrt->mPrintObject->Init(webContainer, aDoc, aIsPrintPreview);
     NS_ENSURE_SUCCESS(rv, rv);
 
-    NS_ENSURE_TRUE(mPrt->mPrintDocList.AppendElement(mPrt->mPrintObject),
+    NS_ENSURE_TRUE(mPrt->mPrintDocList.AppendElement(mPrt->mPrintObject.get()),
                    NS_ERROR_OUT_OF_MEMORY);
 
     mPrt->mIsParentAFrameSet = IsParentAFrameSet(webContainer);
     mPrt->mPrintObject->mFrameType = mPrt->mIsParentAFrameSet ? eFrameSet : eDoc;
 
     // Build the "tree" of PrintObjects
     BuildDocTree(mPrt->mPrintObject->mDocShell, &mPrt->mPrintDocList,
-                 mPrt->mPrintObject);
+                 mPrt->mPrintObject.get());
   }
 
   if (!aIsPrintPreview) {
     SetIsPrinting(true);
   }
 
   // XXX This isn't really correct...
   if (!mPrt->mPrintObject->mDocument ||
       !mPrt->mPrintObject->mDocument->GetRootElement())
     return NS_ERROR_GFX_PRINTER_STARTDOC;
 
   // Create the linkage from the sub-docs back to the content element
   // in the parent document
-  MapContentToWebShells(mPrt->mPrintObject, mPrt->mPrintObject);
+  MapContentToWebShells(mPrt->mPrintObject.get(), mPrt->mPrintObject.get());
 
   mPrt->mIsIFrameSelected = IsThereAnIFrameSelected(webContainer, mPrt->mCurrentFocusWin, mPrt->mIsParentAFrameSet);
 
   // Setup print options for UI
   if (mPrt->mIsParentAFrameSet) {
     if (mPrt->mCurrentFocusWin) {
       mPrt->mPrintSettings->SetHowToEnableFrameUI(nsIPrintSettings::kFrameEnableAll);
     } else {
@@ -717,17 +716,17 @@ nsPrintEngine::DoCommonPrint(bool       
         }
       }
     } else {
       mPrt->mPrintSettings->GetPrintFrameType(&mPrt->mPrintFrameType);
     }
   }
 
   if (mPrt->mPrintFrameType == nsIPrintSettings::kEachFrameSep) {
-    CheckForChildFrameSets(mPrt->mPrintObject);
+    CheckForChildFrameSets(mPrt->mPrintObject.get());
   }
 
   if (NS_FAILED(EnablePOsForPrinting())) {
     return NS_ERROR_FAILURE;
   }
 
   // Attach progressListener to catch network requests.
   nsCOMPtr<nsIWebProgress> webProgress = do_QueryInterface(mPrt->mPrintObject->mDocShell);
@@ -874,17 +873,17 @@ nsPrintEngine::GetPrintPreviewNumPages(i
   // When calling this function, the FinishPrintPreview() function might not
   // been called as there are still some
   if (mPrtPreview) {
     prt = mPrtPreview.get();
   } else {
     prt = mPrt.get();
   }
   if ((!prt) ||
-      NS_FAILED(GetSeqFrameAndCountPagesInternal(prt->mPrintObject, seqFrame, *aPrintPreviewNumPages))) {
+      NS_FAILED(GetSeqFrameAndCountPagesInternal(prt->mPrintObject.get(), seqFrame, *aPrintPreviewNumPages))) {
     return NS_ERROR_FAILURE;
   }
   return NS_OK;
 }
 
 //----------------------------------------------------------------------------------
 // Enumerate all the documents for their titles
 NS_IMETHODIMP
@@ -1074,17 +1073,17 @@ nsPrintEngine::ShowPrintProgress(bool aI
                                                      getter_AddRefs(mPrt->mPrintProgressParams),
                                                      &aDoNotify);
       if (NS_SUCCEEDED(rv)) {
         if (printProgressListener) {
           mPrt->mPrintProgressListeners.AppendObject(printProgressListener);
         }
 
         if (mPrt->mPrintProgressParams) {
-          SetDocAndURLIntoProgress(mPrt->mPrintObject, mPrt->mPrintProgressParams);
+          SetDocAndURLIntoProgress(mPrt->mPrintObject.get(), mPrt->mPrintProgressParams);
         }
       }
     }
   }
 }
 
 //---------------------------------------------------------------------
 bool
@@ -1374,17 +1373,17 @@ nsPrintEngine::MapContentForPO(nsPrintOb
 bool
 nsPrintEngine::IsThereAnIFrameSelected(nsIDocShell* aDocShell,
                                        nsPIDOMWindowOuter* aDOMWin,
                                        bool& aIsParentFrameSet)
 {
   aIsParentFrameSet = IsParentAFrameSet(aDocShell);
   bool iFrameIsSelected = false;
   if (mPrt && mPrt->mPrintObject) {
-    nsPrintObject* po = FindPrintObjectByDOMWin(mPrt->mPrintObject, aDOMWin);
+    nsPrintObject* po = FindPrintObjectByDOMWin(mPrt->mPrintObject.get(), aDOMWin);
     iFrameIsSelected = po && po->mFrameType == eIFrame;
   } else {
     // First, check to see if we are a frameset
     if (!aIsParentFrameSet) {
       // Check to see if there is a currenlt focused frame
       // if so, it means the selected frame is either the main docshell
       // or an IFRAME
       if (aDOMWin) {
@@ -1478,17 +1477,17 @@ nsPrintEngine::GetDisplayTitleAndURL(nsP
     }
   }
 }
 
 //---------------------------------------------------------------------
 nsresult nsPrintEngine::DocumentReadyForPrinting()
 {
   if (mPrt->mPrintFrameType == nsIPrintSettings::kEachFrameSep) {
-    CheckForChildFrameSets(mPrt->mPrintObject);
+    CheckForChildFrameSets(mPrt->mPrintObject.get());
   }
 
   //
   // Send the document to the printer...
   //
   nsresult rv = SetupToPrintContent();
   if (NS_FAILED(rv)) {
     // The print job was canceled or there was a problem
@@ -1717,17 +1716,17 @@ nsPrintEngine::SetupToPrintContent()
     // On some platforms The BeginDocument needs to know the name of the file.
     char16_t* fileName = nullptr;
     mPrt->mPrintSettings->GetToFileName(&fileName);
     fileNameStr = fileName;
   }
 
   nsAutoString docTitleStr;
   nsAutoString docURLStr;
-  GetDisplayTitleAndURL(mPrt->mPrintObject, docTitleStr, docURLStr, eDocTitleDefURLDoc);
+  GetDisplayTitleAndURL(mPrt->mPrintObject.get(), docTitleStr, docURLStr, eDocTitleDefURLDoc);
 
   int32_t startPage = 1;
   int32_t endPage   = mPrt->mNumPrintablePages;
 
   int16_t printRangeType = nsIPrintSettings::kRangeAllPages;
   mPrt->mPrintSettings->GetPrintRange(&printRangeType);
   if (printRangeType == nsIPrintSettings::kRangeSpecifiedPageRange) {
     mPrt->mPrintSettings->GetStartPageRange(&startPage);
@@ -1762,17 +1761,17 @@ nsPrintEngine::SetupToPrintContent()
   NS_ENSURE_SUCCESS(rv, rv);
 
   // This will print the docshell document
   // when it completes asynchronously in the DonePrintingPages method
   // it will check to see if there are more docshells to be printed and
   // then PrintDocContent will be called again.
 
   if (mIsDoingPrinting) {
-    PrintDocContent(mPrt->mPrintObject, rv); // ignore return value
+    PrintDocContent(mPrt->mPrintObject.get(), rv); // ignore return value
   }
 
   return rv;
 }
 
 //-------------------------------------------------------
 // Recursively reflow each sub-doc and then calc
 // all the frame locations of the sub-docs
@@ -1817,17 +1816,17 @@ nsPrintEngine::FirePrintPreviewUpdateEve
     )->RunDOMEventWhenSafe();
   }
 }
 
 nsresult
 nsPrintEngine::InitPrintDocConstruction(bool aHandleError)
 {
   nsresult rv;
-  rv = ReflowDocList(mPrt->mPrintObject, DoSetPixelScale());
+  rv = ReflowDocList(mPrt->mPrintObject.get(), DoSetPixelScale());
   NS_ENSURE_SUCCESS(rv, rv);
 
   FirePrintPreviewUpdateEvent();
 
   if (mLoadCounter == 0) {
     AfterNetworkPrint(aHandleError);
   }
   return rv;
@@ -3057,17 +3056,17 @@ nsPrintEngine::DonePrintingPages(nsPrint
   if (mPageSeqFrame.IsAlive()) {
     nsIPageSequenceFrame* pageSeqFrame = do_QueryFrame(mPageSeqFrame.GetFrame());
     pageSeqFrame->ResetPrintCanvasList();
   }
 
   if (aPO && !mPrt->mIsAborted) {
     aPO->mHasBeenPrinted = true;
     nsresult rv;
-    bool didPrint = PrintDocContent(mPrt->mPrintObject, rv);
+    bool didPrint = PrintDocContent(mPrt->mPrintObject.get(), rv);
     if (NS_SUCCEEDED(rv) && didPrint) {
       PR_PL(("****** In DV::DonePrintingPages PO: %p (%s) didPrint:%s (Not Done Printing)\n", aPO, gFrameTypesStr[aPO->mFrameType], PRT_YESNO(didPrint)));
       return false;
     }
   }
 
   if (NS_SUCCEEDED(aResult)) {
     FirePrintCompletionEvent();
@@ -3167,17 +3166,17 @@ nsPrintEngine::EnablePOsForPrinting()
   //
   // This means there are not FrameSets,
   // but the document could contain an IFrame
   if (printHowEnable == nsIPrintSettings::kFrameEnableNone) {
 
     // Print all the pages or a sub range of pages
     if (printRangeType == nsIPrintSettings::kRangeAllPages ||
         printRangeType == nsIPrintSettings::kRangeSpecifiedPageRange) {
-      SetPrintPO(mPrt->mPrintObject, true);
+      SetPrintPO(mPrt->mPrintObject.get(), true);
 
       // Set the children so they are PrinAsIs
       // In this case, the children are probably IFrames
       if (mPrt->mPrintObject->mKids.Length() > 0) {
         for (const UniquePtr<nsPrintObject>& po : mPrt->mPrintObject->mKids) {
           NS_ASSERTION(po, "nsPrintObject can't be null!");
           SetPrintAsIs(po.get());
         }
@@ -3193,17 +3192,17 @@ nsPrintEngine::EnablePOsForPrinting()
 
     // This means we are either printed a selected IFrame or
     // we are printing the current selection
     if (printRangeType == nsIPrintSettings::kRangeSelection) {
 
       // If the currentFocusDOMWin can'r be null if something is selected
       if (mPrt->mCurrentFocusWin) {
         // Find the selected IFrame
-        nsPrintObject * po = FindPrintObjectByDOMWin(mPrt->mPrintObject, mPrt->mCurrentFocusWin);
+        nsPrintObject * po = FindPrintObjectByDOMWin(mPrt->mPrintObject.get(), mPrt->mCurrentFocusWin);
         if (po != nullptr) {
           mPrt->mSelectedPO = po;
           // Makes sure all of its children are be printed "AsIs"
           SetPrintAsIs(po);
 
           // Now, only enable this POs (the selected PO) and all of its children
           SetPrintPO(po, true);
 
@@ -3241,17 +3240,17 @@ nsPrintEngine::EnablePOsForPrinting()
     }
   }
 
   // check to see if there is a selection when a FrameSet is present
   if (printRangeType == nsIPrintSettings::kRangeSelection) {
     // If the currentFocusDOMWin can'r be null if something is selected
     if (mPrt->mCurrentFocusWin) {
       // Find the selected IFrame
-      nsPrintObject * po = FindPrintObjectByDOMWin(mPrt->mPrintObject, mPrt->mCurrentFocusWin);
+      nsPrintObject * po = FindPrintObjectByDOMWin(mPrt->mPrintObject.get(), mPrt->mCurrentFocusWin);
       if (po != nullptr) {
         mPrt->mSelectedPO = po;
         // Makes sure all of its children are be printed "AsIs"
         SetPrintAsIs(po);
 
         // Now, only enable this POs (the selected PO) and all of its children
         SetPrintPO(po, true);
 
@@ -3272,28 +3271,28 @@ nsPrintEngine::EnablePOsForPrinting()
         PR_PL(("PrintRange:         %s \n", gPrintRangeStr[printRangeType]));
         return NS_OK;
       }
     }
   }
 
   // If we are printing "AsIs" then sets all the POs to be printed as is
   if (mPrt->mPrintFrameType == nsIPrintSettings::kFramesAsIs) {
-    SetPrintAsIs(mPrt->mPrintObject);
-    SetPrintPO(mPrt->mPrintObject, true);
+    SetPrintAsIs(mPrt->mPrintObject.get());
+    SetPrintPO(mPrt->mPrintObject.get(), true);
     return NS_OK;
   }
 
   // If we are printing the selected Frame then
   // find that PO for that selected DOMWin and set it all of its
   // children to be printed
   if (mPrt->mPrintFrameType == nsIPrintSettings::kSelectedFrame) {
 
     if ((mPrt->mIsParentAFrameSet && mPrt->mCurrentFocusWin) || mPrt->mIsIFrameSelected) {
-      nsPrintObject * po = FindPrintObjectByDOMWin(mPrt->mPrintObject, mPrt->mCurrentFocusWin);
+      nsPrintObject * po = FindPrintObjectByDOMWin(mPrt->mPrintObject.get(), mPrt->mCurrentFocusWin);
       if (po != nullptr) {
         mPrt->mSelectedPO = po;
         // NOTE: Calling this sets the "po" and
         // we don't want to do this for documents that have no children,
         // because then the "DoEndPage" gets called and it shouldn't
         if (po->mKids.Length() > 0) {
           // Makes sure that itself, and all of its children are printed "AsIs"
           SetPrintAsIs(po);
@@ -3304,17 +3303,17 @@ nsPrintEngine::EnablePOsForPrinting()
       }
     }
     return NS_OK;
   }
 
   // If we are print each subdoc separately,
   // then don't print any of the FraneSet Docs
   if (mPrt->mPrintFrameType == nsIPrintSettings::kEachFrameSep) {
-    SetPrintPO(mPrt->mPrintObject, true);
+    SetPrintPO(mPrt->mPrintObject.get(), true);
     int32_t cnt = mPrt->mPrintDocList.Length();
     for (int32_t i=0;i<cnt;i++) {
       nsPrintObject* po = mPrt->mPrintDocList.ElementAt(i);
       NS_ASSERTION(po, "nsPrintObject can't be null!");
       if (po->mFrameType == eFrameSet) {
         po->mDontPrint = true;
       }
     }
@@ -3903,18 +3902,18 @@ static void DumpPrintObjectsListStart(co
   NS_ASSERTION(aStr, "Pointer is null!");
   NS_ASSERTION(aDocList, "Pointer is null!");
 
   PR_PL(("%s\n", aStr));
   DumpPrintObjectsList(aDocList);
 }
 
 #define DUMP_DOC_LIST(_title) DumpPrintObjectsListStart((_title), mPrt->mPrintDocList);
-#define DUMP_DOC_TREE DumpPrintObjectsTree(mPrt->mPrintObject);
-#define DUMP_DOC_TREELAYOUT DumpPrintObjectsTreeLayout(mPrt->mPrintObject, mPrt->mPrintDC);
+#define DUMP_DOC_TREE DumpPrintObjectsTree(mPrt->mPrintObject.get());
+#define DUMP_DOC_TREELAYOUT DumpPrintObjectsTreeLayout(mPrt->mPrintObject.get(), mPrt->mPrintDC);
 
 #else
 #define DUMP_DOC_LIST(_title)
 #define DUMP_DOC_TREE
 #define DUMP_DOC_TREELAYOUT
 #endif
 
 //---------------------------------------------------------------