--- a/editor/libeditor/HTMLEditRules.cpp
+++ b/editor/libeditor/HTMLEditRules.cpp
@@ -3459,17 +3459,18 @@ nsresult
 HTMLEditRules::WillMakeDefListItem(Selection* aSelection,
                                    const nsAString *aItemType,
                                    bool aEntireList,
                                    bool* aCancel,
                                    bool* aHandled)
 {
   // for now we let WillMakeList handle this
   NS_NAMED_LITERAL_STRING(listType, "dl");
-  return WillMakeList(aSelection, &listType, aEntireList, nullptr, aCancel, aHandled, aItemType);
+  return WillMakeList(aSelection, &listType.AsString(), aEntireList, nullptr,
+                      aCancel, aHandled, aItemType);
 }
 
 nsresult
 HTMLEditRules::WillMakeBasicBlock(Selection& aSelection,
                                   const nsAString& aBlockType,
                                   bool* aCancel,
                                   bool* aHandled)
 {

--- a/editor/libeditor/HTMLStyleEditor.cpp
+++ b/editor/libeditor/HTMLStyleEditor.cpp
@@ -1628,18 +1628,18 @@ HTMLEditor::GetFontFaceState(bool* aMixe
   NS_ENSURE_TRUE(aMixed, NS_ERROR_FAILURE);
   *aMixed = true;
   outFace.Truncate();
 
   bool first, any, all;
 
   NS_NAMED_LITERAL_STRING(attr, "face");
   nsresult rv =
-    GetInlinePropertyBase(*nsGkAtoms::font, &attr, nullptr, &first, &any,
-                          &all, &outFace);
+    GetInlinePropertyBase(*nsGkAtoms::font, &attr.AsString(), nullptr, &first,
+                          &any, &all, &outFace);
   NS_ENSURE_SUCCESS(rv, rv);
   if (any && !all) {
     return NS_OK; // mixed
   }
   if (all) {
     *aMixed = false;
     return NS_OK;
   }
@@ -1671,18 +1671,18 @@ HTMLEditor::GetFontColorState(bool* aMix
   NS_ENSURE_TRUE(aMixed, NS_ERROR_NULL_POINTER);
   *aMixed = true;
   aOutColor.Truncate();
 
   NS_NAMED_LITERAL_STRING(colorStr, "color");
   bool first, any, all;
 
   nsresult rv =
-    GetInlinePropertyBase(*nsGkAtoms::font, &colorStr, nullptr, &first,
-                          &any, &all, &aOutColor);
+    GetInlinePropertyBase(*nsGkAtoms::font, &colorStr.AsString(), nullptr,
+                          &first, &any, &all, &aOutColor);
   NS_ENSURE_SUCCESS(rv, rv);
   if (any && !all) {
     return NS_OK; // mixed
   }
   if (all) {
     *aMixed = false;
     return NS_OK;
   }

--- a/editor/libeditor/TextEditRules.cpp
+++ b/editor/libeditor/TextEditRules.cpp
@@ -380,18 +380,19 @@ TextEditRules::WillInsertBreak(Selection
   if (IsSingleLineEditor()) {
     *aCancel = true;
   } else {
     // handle docs with a max length
     // NOTE, this function copies inString into outString for us.
     NS_NAMED_LITERAL_STRING(inString, "\n");
     nsAutoString outString;
     bool didTruncate;
-    nsresult rv = TruncateInsertionIfNeeded(aSelection, &inString, &outString,
-                                            aMaxLength, &didTruncate);
+    nsresult rv = TruncateInsertionIfNeeded(aSelection, &inString.AsString(),
+                                            &outString, aMaxLength,
+                                            &didTruncate);
     NS_ENSURE_SUCCESS(rv, rv);
     if (didTruncate) {
       *aCancel = true;
       return NS_OK;
     }
 
     *aCancel = false;
 

--- a/xpcom/string/nsTLiteralString.h
+++ b/xpcom/string/nsTLiteralString.h
@@ -1,44 +1,59 @@
 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
 /* 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/. */
 
-
 /**
  * nsTLiteralString_CharT
  *
  * Stores a null-terminated, immutable sequence of characters.
  *
- * Subclass of nsTString that restricts string value to a literal
- * character sequence.  This class does not own its data. The data is
- * assumed to be permanent. In practice this is true because this code
- * is only usable by and for libxul.
+ * nsTString-lookalike that restricts its string value to a literal character
+ * sequence. Can be implicitly cast to const nsTString& (the const is
+ * essential, since this class's data are not writable). The data are assumed
+ * to be static (permanent) and therefore, as an optimization, this class
+ * does not have a destructor.
  */
-class nsTLiteralString_CharT : public nsTString_CharT
+class nsTLiteralString_CharT : public mozilla::detail::nsTStringRepr_CharT
 {
 public:
 
   typedef nsTLiteralString_CharT self_type;
 
 public:
 
   /**
    * constructor
    */
 
   template<size_type N>
   explicit nsTLiteralString_CharT(const char_type (&aStr)[N])
-    : string_type(const_cast<char_type*>(aStr), N - 1, F_TERMINATED | F_LITERAL)
+    : base_string_type(const_cast<char_type*>(aStr), N - 1, F_TERMINATED | F_LITERAL)
   {
   }
 
   /**
+   * For compatibility with existing code that requires const ns[C]String*.
+   * Use sparingly. If possible, rewrite code to use const ns[C]String&
+   * and the implicit cast will just work.
+   */
+  const nsTString_CharT& AsString() const
+  {
+    return *reinterpret_cast<const nsTString_CharT*>(this);
+  }
+
+  operator const nsTString_CharT&() const
+  {
+    return AsString();
+  }
+
+  /**
    * Prohibit get() on temporaries as in nsLiteralCString("x").get().
    * These should be written as just "x", using a string literal directly.
    */
 #if defined(CharT_is_PRUnichar) && defined(MOZ_USE_CHAR16_WRAPPER)
   char16ptr_t get() const && = delete;
   char16ptr_t get() const &
 #else
   const char_type* get() const && = delete;
@@ -48,9 +63,15 @@ public:
     return mData;
   }
 
 private:
 
   // NOT TO BE IMPLEMENTED
   template<size_type N>
   nsTLiteralString_CharT(char_type (&aStr)[N]) = delete;
+
+  self_type& operator=(const self_type&) = delete;
 };
+
+static_assert(sizeof(nsTLiteralString_CharT) == sizeof(nsTString_CharT),
+              "nsTLiteralString_CharT can masquerade as nsTString_CharT, "
+              "so they must have identical layout");