--- a/layout/base/AccessibleCaret.cpp
+++ b/layout/base/AccessibleCaret.cpp
@@ -137,17 +137,17 @@ AccessibleCaret::SetSelectionBarEnabled(
   ErrorResult rv;
   CaretElement()->ClassList()->Toggle(NS_LITERAL_STRING("no-bar"),
                                       Optional<bool>(!aEnabled), rv);
   MOZ_ASSERT(!rv.Failed());
 
   mSelectionBarEnabled = aEnabled;
 }
 
-/* static */ nsString
+/* static */ nsAutoString
 AccessibleCaret::AppearanceString(Appearance aAppearance)
 {
   nsAutoString string;
   switch (aAppearance) {
   case Appearance::None:
   case Appearance::NormalNotShown:
     string = NS_LITERAL_STRING("none");
     break;

--- a/layout/base/AccessibleCaret.h
+++ b/layout/base/AccessibleCaret.h
@@ -176,18 +176,18 @@ protected:
 
   nsIFrame* RootFrame() const
   {
     return mPresShell->GetRootFrame();
   }
 
   nsIFrame* CustomContentContainerFrame() const;
 
-  // Transform Appearance to CSS class name in ua.css.
-  static nsString AppearanceString(Appearance aAppearance);
+  // Transform Appearance to CSS id used in ua.css.
+  static nsAutoString AppearanceString(Appearance aAppearance);
 
   already_AddRefed<dom::Element> CreateCaretElement(nsIDocument* aDocument) const;
 
   // Inject caret element into custom content container.
   void InjectCaretElement(nsIDocument* aDocument);
 
   // Remove caret element from custom content container.
   void RemoveCaretElement(nsIDocument* aDocument);