--- a/dom/base/nsDocumentEncoder.cpp
+++ b/dom/base/nsDocumentEncoder.cpp
@@ -199,17 +199,18 @@ NS_IMPL_CYCLE_COLLECTING_ADDREF(nsDocume
 NS_IMPL_CYCLE_COLLECTING_RELEASE(nsDocumentEncoder)
 
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(nsDocumentEncoder)
    NS_INTERFACE_MAP_ENTRY(nsIDocumentEncoder)
    NS_INTERFACE_MAP_ENTRY(nsISupports)
 NS_INTERFACE_MAP_END
 
 NS_IMPL_CYCLE_COLLECTION(nsDocumentEncoder,
-                         mDocument, mSelection, mRange, mNode, mCommonParent)
+                         mDocument, mSelection, mRange, mNode, mSerializer,
+                         mCommonParent)
 
 nsDocumentEncoder::nsDocumentEncoder() : mCachedBuffer(nullptr)
 {
   Initialize();
   mMimeType.AssignLiteral("text/plain");
 }
 
 void nsDocumentEncoder::Initialize(bool aClearCachedSerializer)

--- a/dom/base/nsPlainTextSerializer.cpp
+++ b/dom/base/nsPlainTextSerializer.cpp
@@ -50,16 +50,27 @@ static const  char16_t kSPACE = ' ';
 static int32_t HeaderLevel(nsIAtom* aTag);
 static int32_t GetUnicharWidth(char16_t ucs);
 static int32_t GetUnicharStringWidth(const char16_t* pwcs, int32_t n);
 
 // Someday may want to make this non-const:
 static const uint32_t TagStackSize = 500;
 static const uint32_t OLStackSize = 100;
 
+NS_IMPL_CYCLE_COLLECTING_ADDREF(nsPlainTextSerializer)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(nsPlainTextSerializer)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(nsPlainTextSerializer)
+  NS_INTERFACE_MAP_ENTRY(nsIContentSerializer)
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTION(nsPlainTextSerializer,
+                         mElement)
+
 nsresult
 NS_NewPlainTextSerializer(nsIContentSerializer** aSerializer)
 {
   RefPtr<nsPlainTextSerializer> it = new nsPlainTextSerializer();
   it.forget(aSerializer);
   return NS_OK;
 }
 
@@ -111,20 +122,16 @@ nsPlainTextSerializer::nsPlainTextSerial
 
 nsPlainTextSerializer::~nsPlainTextSerializer()
 {
   delete[] mTagStack;
   delete[] mOLStack;
   NS_WARNING_ASSERTION(mHeadLevel == 0, "Wrong head level!");
 }
 
-NS_IMPL_ISUPPORTS(nsPlainTextSerializer,
-                  nsIContentSerializer)
-
-
 NS_IMETHODIMP 
 nsPlainTextSerializer::Init(uint32_t aFlags, uint32_t aWrapColumn,
                             const char* aCharSet, bool aIsCopying,
                             bool aIsWholeDocument)
 {
 #ifdef DEBUG
   // Check if the major control flags are set correctly.
   if (aFlags & nsIDocumentEncoder::OutputFormatFlowed) {

--- a/dom/base/nsPlainTextSerializer.h
+++ b/dom/base/nsPlainTextSerializer.h
@@ -32,17 +32,18 @@ class Element;
 } // namespace dom
 } // namespace mozilla
 
 class nsPlainTextSerializer final : public nsIContentSerializer
 {
 public:
   nsPlainTextSerializer();
 
-  NS_DECL_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_CLASS(nsPlainTextSerializer)
 
   // nsIContentSerializer
   NS_IMETHOD Init(uint32_t flags, uint32_t aWrapColumn,
                   const char* aCharSet, bool aIsCopying,
                   bool aIsWholeDocument) override;
 
   NS_IMETHOD AppendText(nsIContent* aText, int32_t aStartOffset,
                         int32_t aEndOffset, nsAString& aStr) override;