--- a/editor/libeditor/EditorBase.cpp
+++ b/editor/libeditor/EditorBase.cpp
@@ -708,17 +708,17 @@ EditorBase::DeleteSelection(EDirection a
 NS_IMETHODIMP
 EditorBase::GetSelection(Selection** aSelection)
 {
   return GetSelection(SelectionType::eNormal, aSelection);
 }
 
 nsresult
 EditorBase::GetSelection(SelectionType aSelectionType,
-                         Selection** aSelection)
+                         Selection** aSelection) const
 {
   NS_ENSURE_TRUE(aSelection, NS_ERROR_NULL_POINTER);
   *aSelection = nullptr;
   nsISelectionController* selcon = GetSelectionController();
   if (!selcon) {
     return NS_ERROR_NOT_INITIALIZED;
   }
   RefPtr<Selection> selection =

--- a/editor/libeditor/EditorBase.h
+++ b/editor/libeditor/EditorBase.h
@@ -286,20 +286,20 @@ public:
     if (!presShell) {
       return nullptr;
     }
     nsISelectionController* sc = static_cast<PresShell*>(presShell);
     return sc;
   }
 
   nsresult GetSelection(SelectionType aSelectionType,
-                        Selection** aSelection);
+                        Selection** aSelection) const;
 
   Selection* GetSelection(SelectionType aSelectionType =
-                                          SelectionType::eNormal)
+                                          SelectionType::eNormal) const
   {
     nsISelectionController* sc = GetSelectionController();
     if (!sc) {
       return nullptr;
     }
     Selection* selection = sc->GetSelection(ToRawSelectionType(aSelectionType));
     return selection;
   }

--- a/editor/libeditor/TextEditor.cpp
+++ b/editor/libeditor/TextEditor.cpp
@@ -1707,21 +1707,20 @@ TextEditor::CanCopy(bool* aCanCopy)
 NS_IMETHODIMP
 TextEditor::CanDelete(bool* aCanDelete)
 {
   NS_ENSURE_ARG_POINTER(aCanDelete);
   *aCanDelete = IsModifiable() && CanCutOrCopy(ePasswordFieldAllowed);
   return NS_OK;
 }
 
-// Used by OutputToString
 already_AddRefed<nsIDocumentEncoder>
 TextEditor::GetAndInitDocEncoder(const nsAString& aFormatType,
-                                 uint32_t aFlags,
-                                 const nsACString& aCharset)
+                                 uint32_t aDocumentEncoderFlags,
+                                 const nsACString& aCharset) const
 {
   nsCOMPtr<nsIDocumentEncoder> docEncoder;
   if (!mCachedDocumentEncoder ||
       !mCachedDocumentEncoderType.Equals(aFormatType)) {
     nsAutoCString formatType(NS_DOC_ENCODER_CONTRACTID_BASE);
     LossyAppendUTF16toASCII(aFormatType, formatType);
     docEncoder = do_CreateInstance(formatType.get());
     if (NS_WARN_IF(!docEncoder)) {
@@ -1734,34 +1733,35 @@ TextEditor::GetAndInitDocEncoder(const n
   }
 
   nsCOMPtr<nsIDocument> doc = GetDocument();
   NS_ASSERTION(doc, "Need a document");
 
   nsresult rv =
     docEncoder->NativeInit(
                   doc, aFormatType,
-                  aFlags | nsIDocumentEncoder::RequiresReinitAfterOutput);
+                  aDocumentEncoderFlags |
+                    nsIDocumentEncoder::RequiresReinitAfterOutput);
   if (NS_WARN_IF(NS_FAILED(rv))) {
     return nullptr;
   }
 
   if (!aCharset.IsEmpty() && !aCharset.EqualsLiteral("null")) {
     docEncoder->SetCharset(aCharset);
   }
 
   int32_t wrapWidth = WrapWidth();
   if (wrapWidth >= 0) {
     Unused << docEncoder->SetWrapColumn(wrapWidth);
   }
 
   // Set the selection, if appropriate.
   // We do this either if the OutputSelectionOnly flag is set,
   // in which case we use our existing selection ...
-  if (aFlags & nsIDocumentEncoder::OutputSelectionOnly) {
+  if (aDocumentEncoderFlags & nsIDocumentEncoder::OutputSelectionOnly) {
     RefPtr<Selection> selection = GetSelection();
     if (NS_WARN_IF(!selection)) {
       return nullptr;
     }
     rv = docEncoder->SetSelection(selection);
     if (NS_WARN_IF(NS_FAILED(rv))) {
       return nullptr;
     }
@@ -1779,17 +1779,16 @@ TextEditor::GetAndInitDocEncoder(const n
         return nullptr;
       }
     }
   }
 
   return docEncoder.forget();
 }
 
-
 NS_IMETHODIMP
 TextEditor::OutputToString(const nsAString& aFormatType,
                            uint32_t aFlags,
                            nsAString& aOutputString)
 {
   // Protect the edit rules object from dying
   RefPtr<TextEditRules> rules(mRules);
 

--- a/editor/libeditor/TextEditor.h
+++ b/editor/libeditor/TextEditor.h
@@ -322,20 +322,29 @@ protected: // Shouldn't be used by frien
    * Return true if the data is safe to insert as the source and destination
    * principals match, or we are in a editor context where this doesn't matter.
    * Otherwise, the data must be sanitized first.
    */
   bool IsSafeToInsertData(nsIDocument* aSourceDoc);
 
   virtual nsresult InitRules();
 
-  already_AddRefed<nsIDocumentEncoder> GetAndInitDocEncoder(
-                                         const nsAString& aFormatType,
-                                         uint32_t aFlags,
-                                         const nsACString& aCharset);
+  /**
+   * GetAndInitDocEncoder() returns a document encoder instance for aFormatType
+   * after initializing it.  The result may be cached for saving recreation
+   * cost.
+   *
+   * @param aFormatType             MIME type like "text/plain".
+   * @param aDocumentEncoderFlags   Flags of nsIDocumentEncoder.
+   * @param aCharset                Encoding of the document.
+   */
+  already_AddRefed<nsIDocumentEncoder>
+  GetAndInitDocEncoder(const nsAString& aFormatType,
+                       uint32_t aDocumentEncoderFlags,
+                       const nsACString& aCharset) const;
 
   /**
    * Factored methods for handling insertion of data from transferables
    * (drag&drop or clipboard).
    */
   virtual nsresult PrepareTransferable(nsITransferable** transferable);
 
   nsresult InsertTextFromTransferable(nsITransferable* transferable);
@@ -388,18 +397,18 @@ protected: // Shouldn't be used by frien
    *                    a composition event handler in web contents moved focus
    *                    for committing the composition, returns false.
    */
   bool EnsureComposition(WidgetCompositionEvent& aCompositionEvent);
 
   virtual already_AddRefed<nsIContent> GetInputEventTargetContent() override;
 
 protected:
-  nsCOMPtr<nsIDocumentEncoder> mCachedDocumentEncoder;
-  nsString mCachedDocumentEncoderType;
+  mutable nsCOMPtr<nsIDocumentEncoder> mCachedDocumentEncoder;
+  mutable nsString mCachedDocumentEncoderType;
   int32_t mWrapColumn;
   int32_t mMaxTextLength;
   int32_t mInitTriggerCounter;
   int32_t mNewlineHandling;
   int32_t mCaretStyle;
 
   friend class AutoEditInitRulesTrigger;
   friend class TextEditRules;