--- a/editor/libeditor/EditorBase.cpp
+++ b/editor/libeditor/EditorBase.cpp
@@ -790,26 +790,16 @@ EditorBase::GetTransactionManager(nsITra
   *aTxnManager = nullptr;
   NS_ENSURE_TRUE(mTxnMgr, NS_ERROR_FAILURE);
 
   NS_ADDREF(*aTxnManager = mTxnMgr);
   return NS_OK;
 }
 
 NS_IMETHODIMP
-EditorBase::SetTransactionManager(nsITransactionManager* aTxnManager)
-{
-  NS_ENSURE_TRUE(aTxnManager, NS_ERROR_FAILURE);
-
-  // nsITransactionManager is builtinclass, so this is safe
-  mTxnMgr = static_cast<nsTransactionManager*>(aTxnManager);
-  return NS_OK;
-}
-
-NS_IMETHODIMP
 EditorBase::Undo(uint32_t aCount)
 {
   ForceCompositionEnd();
 
   bool hasTxnMgr, hasTransaction = false;
   CanUndo(&hasTxnMgr, &hasTransaction);
   NS_ENSURE_TRUE(hasTransaction, NS_OK);
 

--- a/editor/nsIEditor.idl
+++ b/editor/nsIEditor.idl
@@ -167,17 +167,17 @@ interface nsIEditor  : nsISupports
     *                    to increase or decrease the count
     */
   void incrementModificationCount(in long aModCount);
 
   /* ------------ Transaction methods -------------- */
 
   /** transactionManager Get the transaction manager the editor is using.
     */
-  attribute nsITransactionManager transactionManager;
+  readonly attribute nsITransactionManager transactionManager;
 
   /** doTransaction() fires a transaction.
     * It is provided here so clients can create their own transactions.
     * If a transaction manager is present, it is used.
     * Otherwise, the transaction is just executed directly.
     *
     * @param aTxn the transaction to execute
     */