--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -9387,18 +9387,18 @@ nsDocShell::SetupNewViewer(nsIContentVie
   // Init settings
   DoGetPositionAndSize(&x, &y, &cx, &cy);
 
   nsCOMPtr<nsIDocShellTreeItem> parentAsItem;
   NS_ENSURE_SUCCESS(GetSameTypeParent(getter_AddRefs(parentAsItem)),
                     NS_ERROR_FAILURE);
   nsCOMPtr<nsIDocShell> parent(do_QueryInterface(parentAsItem));
 
-  nsAutoCString forceCharset;
-  nsAutoCString hintCharset;
+  const Encoding* forceCharset = nullptr;
+  const Encoding* hintCharset = nullptr;
   int32_t hintCharsetSource;
   int32_t minFontSize;
   float textZoom;
   float pageZoom;
   float overrideDPPX;
   bool styleDisabled;
   // |newMUDV| also serves as a flag to set the data from the above vars
   nsCOMPtr<nsIContentViewer> newCv;
@@ -9423,20 +9423,18 @@ nsDocShell::SetupNewViewer(nsIContentVie
     } else {
       // No old content viewer, so get state from parent's content viewer
       parent->GetContentViewer(getter_AddRefs(oldCv));
     }
 
     if (oldCv) {
       newCv = aNewViewer;
       if (newCv) {
-        NS_ENSURE_SUCCESS(oldCv->GetForceCharacterSet(forceCharset),
-                          NS_ERROR_FAILURE);
-        NS_ENSURE_SUCCESS(oldCv->GetHintCharacterSet(hintCharset),
-                          NS_ERROR_FAILURE);
+        forceCharset = oldCv->GetForceCharset();
+        hintCharset = oldCv->GetHintCharset();
         NS_ENSURE_SUCCESS(oldCv->GetHintCharacterSetSource(&hintCharsetSource),
                           NS_ERROR_FAILURE);
         NS_ENSURE_SUCCESS(oldCv->GetMinFontSize(&minFontSize),
                           NS_ERROR_FAILURE);
         NS_ENSURE_SUCCESS(oldCv->GetTextZoom(&textZoom),
                           NS_ERROR_FAILURE);
         NS_ENSURE_SUCCESS(oldCv->GetFullZoom(&pageZoom),
                           NS_ERROR_FAILURE);
@@ -9493,20 +9491,18 @@ nsDocShell::SetupNewViewer(nsIContentVie
     mContentViewer = nullptr;
     NS_WARNING("ContentViewer Initialization failed");
     return NS_ERROR_FAILURE;
   }
 
   // If we have old state to copy, set the old state onto the new content
   // viewer
   if (newCv) {
-    NS_ENSURE_SUCCESS(newCv->SetForceCharacterSet(forceCharset),
-                      NS_ERROR_FAILURE);
-    NS_ENSURE_SUCCESS(newCv->SetHintCharacterSet(hintCharset),
-                      NS_ERROR_FAILURE);
+    newCv->SetForceCharset(forceCharset);
+    newCv->SetHintCharset(hintCharset);
     NS_ENSURE_SUCCESS(newCv->SetHintCharacterSetSource(hintCharsetSource),
                       NS_ERROR_FAILURE);
     NS_ENSURE_SUCCESS(newCv->SetMinFontSize(minFontSize),
                       NS_ERROR_FAILURE);
     NS_ENSURE_SUCCESS(newCv->SetTextZoom(textZoom),
                       NS_ERROR_FAILURE);
     NS_ENSURE_SUCCESS(newCv->SetFullZoom(pageZoom),
                       NS_ERROR_FAILURE);

--- a/docshell/base/nsIContentViewer.idl
+++ b/docshell/base/nsIContentViewer.idl
@@ -16,25 +16,29 @@ interface nsIPrintSettings;
 #include "nsTArray.h"
 #include "nsRect.h"
 
 class nsIWidget;
 class nsIPresShell;
 class nsPresContext;
 class nsView;
 class nsDOMNavigationTiming;
+namespace mozilla {
+class Encoding;
+}
 %}
 
 [ptr] native nsIWidgetPtr(nsIWidget);
 [ref] native nsIntRectRef(nsIntRect);
 [ptr] native nsIPresShellPtr(nsIPresShell);
 [ptr] native nsPresContextPtr(nsPresContext);
 [ptr] native nsViewPtr(nsView);
 [ptr] native nsDOMNavigationTimingPtr(nsDOMNavigationTiming);
 [ref] native nsIContentViewerTArray(nsTArray<nsCOMPtr<nsIContentViewer> >);
+[ptr] native Encoding(const mozilla::Encoding);
 
 [scriptable, builtinclass, uuid(2da17016-7851-4a45-a7a8-00b360e01595)]
 interface nsIContentViewer : nsISupports
 {
   [noscript] void init(in nsIWidgetPtr aParentWidget,
                        [const] in nsIntRectRef aBounds);
 
   attribute nsIDocShell container;
@@ -277,9 +281,14 @@ interface nsIContentViewer : nsISupports
    * @param mediaType The media type to be emulated
    */
   void emulateMedium(in AString aMediaType);
 
   /*
    * Restore the viewer's natural media type
    */
   void stopEmulatingMedium();
+
+  [noscript, notxpcom] Encoding getHintCharset();
+  [noscript, notxpcom] void setHintCharset(in Encoding aEncoding);
+  [noscript, notxpcom] Encoding getForceCharset();
+  [noscript, notxpcom] void setForceCharset(in Encoding aEncoding);
 };

--- a/dom/html/nsHTMLDocument.cpp
+++ b/dom/html/nsHTMLDocument.cpp
@@ -146,30 +146,16 @@ static bool ConvertToMidasInternalComman
 static bool ConvertToMidasInternalCommand(const nsAString & inCommandID,
                                             nsACString& outCommandID);
 
 // ==================================================================
 // =
 // ==================================================================
 
 static bool
-IsAsciiCompatible(const nsACString& aPreferredName)
-{
-  // HZ and UTF-7 are no longer in mozilla-central, but keeping them here
-  // just in case for the benefit of comm-central.
-  return !(aPreferredName.LowerCaseEqualsLiteral("utf-16") ||
-           aPreferredName.LowerCaseEqualsLiteral("utf-16be") ||
-           aPreferredName.LowerCaseEqualsLiteral("utf-16le") ||
-           aPreferredName.LowerCaseEqualsLiteral("replacement") ||
-           aPreferredName.LowerCaseEqualsLiteral("hz-gb-2312") ||
-           aPreferredName.LowerCaseEqualsLiteral("utf-7") ||
-           aPreferredName.LowerCaseEqualsLiteral("x-imap4-modified-utf7"));
-}
-
-static bool
 IsAsciiCompatible(const Encoding* aEncoding)
 {
   return aEncoding->IsAsciiCompatible() || aEncoding == ISO_2022_JP_ENCODING;
 }
 
 nsresult
 NS_NewHTMLDocument(nsIDocument** aInstancePtrResult, bool aLoadedAsData)
 {
@@ -295,63 +281,56 @@ nsHTMLDocument::TryHintCharset(nsIConten
                                int32_t& aCharsetSource,
                                NotNull<const Encoding*>& aEncoding)
 {
   if (aCv) {
     int32_t requestCharsetSource;
     nsresult rv = aCv->GetHintCharacterSetSource(&requestCharsetSource);
 
     if(NS_SUCCEEDED(rv) && kCharsetUninitialized != requestCharsetSource) {
-      nsAutoCString requestCharset;
-      rv = aCv->GetHintCharacterSet(requestCharset);
+      auto requestCharset = aCv->GetHintCharset();
       aCv->SetHintCharacterSetSource((int32_t)(kCharsetUninitialized));
 
       if (requestCharsetSource <= aCharsetSource)
         return;
 
-      if (NS_SUCCEEDED(rv) && !requestCharset.IsEmpty()) {
-        auto encoding = Encoding::ForName(requestCharset);
-        if (IsAsciiCompatible(encoding)) {
-          aCharsetSource = requestCharsetSource;
-          aEncoding = encoding;
-        }
-        return;
+      if (requestCharset && IsAsciiCompatible(requestCharset)) {
+        aCharsetSource = requestCharsetSource;
+        aEncoding = WrapNotNull(requestCharset);
       }
+      return;
     }
   }
   return;
 }
 
 
 void
 nsHTMLDocument::TryUserForcedCharset(nsIContentViewer* aCv,
                                      nsIDocShell*  aDocShell,
                                      int32_t& aCharsetSource,
                                      NotNull<const Encoding*>& aEncoding)
 {
-  nsresult rv = NS_OK;
-
   if(kCharsetFromUserForced <= aCharsetSource)
     return;
 
   // mCharacterSet not updated yet for channel, so check aEncoding, too.
   if (WillIgnoreCharsetOverride() || !IsAsciiCompatible(aEncoding)) {
     return;
   }
 
-  nsAutoCString forceCharsetFromDocShell;
+  const Encoding* forceCharsetFromDocShell = nullptr;
   if (aCv) {
     // XXX mailnews-only
-    rv = aCv->GetForceCharacterSet(forceCharsetFromDocShell);
+    forceCharsetFromDocShell = aCv->GetForceCharset();
   }
 
-  if(NS_SUCCEEDED(rv) &&
-     !forceCharsetFromDocShell.IsEmpty() &&
+  if(forceCharsetFromDocShell &&
      IsAsciiCompatible(forceCharsetFromDocShell)) {
-    aEncoding = Encoding::ForName(forceCharsetFromDocShell);
+    aEncoding = WrapNotNull(forceCharsetFromDocShell);
     aCharsetSource = kCharsetFromUserForced;
     return;
   }
 
   if (aDocShell) {
     // This is the Character Encoding menu code path in Firefox
     auto encoding = nsDocShell::Cast(aDocShell)->GetForcedCharset();
 

--- a/layout/base/nsDocumentViewer.cpp
+++ b/layout/base/nsDocumentViewer.cpp
@@ -397,19 +397,19 @@ protected:
 
 #ifdef DEBUG
   FILE* mDebugFile;
 #endif // DEBUG
 #endif // NS_PRINTING
 
   /* character set member data */
   int32_t mHintCharsetSource;
-  nsCString mHintCharset;
-  nsCString mForceCharacterSet;
-  
+  const Encoding* mHintCharset;
+  const Encoding* mForceCharacterSet;
+
   bool mIsPageMode;
   bool mInitializedForPrintPreview;
   bool mHidden;
   bool mPrintRelated; // Only use for asserts.
   bool mPresShellDestroyed; // Only use for asserts.
   bool mDestroyWasFull; // Only use for asserts.
 };
 
@@ -537,16 +537,18 @@ nsDocumentViewer::nsDocumentViewer()
     mOriginalPrintPreviewScale(0.0),
     mPrintPreviewZoom(1.0),
 #endif // NS_PRINT_PREVIEW
 #ifdef DEBUG
     mDebugFile(nullptr),
 #endif // DEBUG
 #endif // NS_PRINTING
     mHintCharsetSource(kCharsetUninitialized),
+    mHintCharset(nullptr),
+    mForceCharacterSet(nullptr),
     mIsPageMode(false),
     mInitializedForPrintPreview(false),
     mHidden(false),
     mPrintRelated(false),
     mPresShellDestroyed(true),
     mDestroyWasFull(false)
 {
   PrepareToStartLoad();
@@ -3359,25 +3361,37 @@ nsDocumentViewer::StopEmulatingMedium()
                                           nullptr);
   }
 
   return NS_OK;
 }
 
 NS_IMETHODIMP nsDocumentViewer::GetForceCharacterSet(nsACString& aForceCharacterSet)
 {
-  aForceCharacterSet = mForceCharacterSet;
+  auto encoding = nsDocumentViewer::GetForceCharset();
+  if (encoding) {
+    encoding->Name(aForceCharacterSet);
+  } else {
+    aForceCharacterSet.Truncate();
+  }
   return NS_OK;
 }
 
+/* [noscript,notxpcom] Encoding getForceCharset (); */
+NS_IMETHODIMP_(const Encoding *)
+nsDocumentViewer::GetForceCharset()
+{
+  return mForceCharacterSet;
+}
+
 static void
 SetChildForceCharacterSet(nsIContentViewer* aChild, void* aClosure)
 {
-  const nsACString* charset = static_cast<nsACString*>(aClosure);
-  aChild->SetForceCharacterSet(*charset);
+  auto encoding = static_cast<const Encoding*>(aClosure);
+  aChild->SetForceCharset(encoding);
 }
 
 NS_IMETHODIMP
 nsDocumentViewer::SetForceCharacterSet(const nsACString& aForceCharacterSet)
 {
   // This method is scriptable, so add-ons could pass in something other
   // than a canonical name. However, in case where the input is a canonical
   // name, "replacement" doesn't survive label resolution. Additionally, the
@@ -3386,39 +3400,52 @@ nsDocumentViewer::SetForceCharacterSet(c
   if (!aForceCharacterSet.IsEmpty()) {
     if (aForceCharacterSet.EqualsLiteral("replacement")) {
       encoding = REPLACEMENT_ENCODING;
     } else if (!(encoding = Encoding::ForLabel(aForceCharacterSet))) {
       // Reject unknown labels
       return NS_ERROR_INVALID_ARG;
     }
   }
-  if (encoding) {
-    encoding->Name(mForceCharacterSet);
-  } else {
-    mForceCharacterSet.Truncate();
-  }
+  nsDocumentViewer::SetForceCharset(encoding);
+  return NS_OK;
+}
+
+/* [noscript,notxpcom] void setForceCharset (in Encoding aEncoding); */
+NS_IMETHODIMP_(void)
+nsDocumentViewer::SetForceCharset(const Encoding *aEncoding)
+{
+  mForceCharacterSet = aEncoding;
   // now set the force char set on all children of mContainer
-  CallChildren(SetChildForceCharacterSet, (void*) &aForceCharacterSet);
-  return NS_OK;
+  CallChildren(SetChildForceCharacterSet, (void*) aEncoding);
 }
 
 NS_IMETHODIMP nsDocumentViewer::GetHintCharacterSet(nsACString& aHintCharacterSet)
 {
-
-  if(kCharsetUninitialized == mHintCharsetSource) {
-    aHintCharacterSet.Truncate();
+  auto encoding = nsDocumentViewer::GetHintCharset();
+  if (encoding) {
+    encoding->Name(aHintCharacterSet);
   } else {
-    aHintCharacterSet = mHintCharset;
-    // this can't possibly be right.  we can't set a value just because somebody got a related value!
-    //mHintCharsetSource = kCharsetUninitialized;
+    aHintCharacterSet.Truncate();
   }
   return NS_OK;
 }
 
+/* [noscript,notxpcom] Encoding getHintCharset (); */
+NS_IMETHODIMP_(const Encoding *)
+nsDocumentViewer::GetHintCharset()
+{
+  if(kCharsetUninitialized == mHintCharsetSource) {
+    return nullptr;
+  }
+  // this can't possibly be right.  we can't set a value just because somebody got a related value!
+  //mHintCharsetSource = kCharsetUninitialized;
+  return mHintCharset;
+}
+
 NS_IMETHODIMP nsDocumentViewer::GetHintCharacterSetSource(int32_t *aHintCharacterSetSource)
 {
   NS_ENSURE_ARG_POINTER(aHintCharacterSetSource);
 
   *aHintCharacterSetSource = mHintCharsetSource;
   return NS_OK;
 }
 
@@ -3436,18 +3463,18 @@ nsDocumentViewer::SetHintCharacterSetSou
   CallChildren(SetChildHintCharacterSetSource,
                       NS_INT32_TO_PTR(aHintCharacterSetSource));
   return NS_OK;
 }
 
 static void
 SetChildHintCharacterSet(nsIContentViewer* aChild, void* aClosure)
 {
-  const nsACString* charset = static_cast<nsACString*>(aClosure);
-  aChild->SetHintCharacterSet(*charset);
+  auto encoding = static_cast<const Encoding*>(aClosure);
+  aChild->SetHintCharset(encoding);
 }
 
 NS_IMETHODIMP
 nsDocumentViewer::SetHintCharacterSet(const nsACString& aHintCharacterSet)
 {
   // This method is scriptable, so add-ons could pass in something other
   // than a canonical name. However, in case where the input is a canonical
   // name, "replacement" doesn't survive label resolution. Additionally, the
@@ -3456,24 +3483,27 @@ nsDocumentViewer::SetHintCharacterSet(co
   if (!aHintCharacterSet.IsEmpty()) {
     if (aHintCharacterSet.EqualsLiteral("replacement")) {
       encoding = REPLACEMENT_ENCODING;
     } else if (!(encoding = Encoding::ForLabel(aHintCharacterSet))) {
       // Reject unknown labels
       return NS_ERROR_INVALID_ARG;
     }
   }
-  if (encoding) {
-    encoding->Name(mHintCharset);
-  } else {
-    mHintCharset.Truncate();
-  }
+  nsDocumentViewer::SetHintCharset(encoding);
+  return NS_OK;
+}
+
+/* [noscript,notxpcom] void setHintCharset (in Encoding aEncoding); */
+NS_IMETHODIMP_(void)
+nsDocumentViewer::SetHintCharset(const Encoding *aEncoding)
+{
+  mHintCharset = aEncoding;
   // now set the hint char set on all children of mContainer
-  CallChildren(SetChildHintCharacterSet, (void*) &aHintCharacterSet);
-  return NS_OK;
+  CallChildren(SetChildHintCharacterSet, (void*) aEncoding);
 }
 
 static void
 AppendChildSubtree(nsIContentViewer* aChild, void* aClosure)
 {
   nsTArray<nsCOMPtr<nsIContentViewer> >& array =
     *static_cast<nsTArray<nsCOMPtr<nsIContentViewer> >*>(aClosure);
   aChild->AppendSubtree(array);