--- a/dom/html/nsTextEditorState.cpp
+++ b/dom/html/nsTextEditorState.cpp
@@ -2279,21 +2279,18 @@ nsTextEditorState::CreateRootNode()
   nodeInfo = doc->NodeInfoManager()->GetNodeInfo(nsGkAtoms::div, nullptr,
                                                  kNameSpaceID_XHTML,
                                                  nsIDOMNode::ELEMENT_NODE);
 
   nsresult rv = NS_NewHTMLElement(getter_AddRefs(mRootNode), nodeInfo.forget(),
                                   NOT_FROM_PARSER);
   NS_ENSURE_SUCCESS(rv, rv);
 
-  if (!IsSingleLineTextControl()) {
-    mMutationObserver = new nsAnonDivObserver(this);
-    mRootNode->AddMutationObserver(mMutationObserver);
-  }
-
+  mMutationObserver = new nsAnonDivObserver(this);
+  mRootNode->AddMutationObserver(mMutationObserver);
   return rv;
 }
 
 nsresult
 nsTextEditorState::InitializeRootNode()
 {
   // Make our root node editable
   mRootNode->SetFlags(NODE_IS_EDITABLE);
@@ -2430,18 +2427,17 @@ nsTextEditorState::GetValue(nsAString& a
   // been set to the editor yet.
   if (mIsCommittingComposition) {
     aValue = mValueBeingSet;
     return;
   }
 
   if (mTextEditor && mBoundFrame &&
       (mEditorInitialized || !IsSingleLineTextControl())) {
-    bool canCache = aIgnoreWrap && !IsSingleLineTextControl();
-    if (canCache && !mCachedValue.IsEmpty()) {
+    if (aIgnoreWrap && !mCachedValue.IsEmpty()) {
       aValue = mCachedValue;
       return;
     }
 
     aValue.Truncate(); // initialize out param
 
     uint32_t flags = (nsIDocumentEncoder::OutputLFLineBreak |
                       nsIDocumentEncoder::OutputPreformatted |
@@ -2470,17 +2466,19 @@ nsTextEditorState::GetValue(nsAString& a
     // if plaintext editor didn't create <br> nodes all over), we wouldn't need
     // this.
     { /* Scope for AutoNoJSAPI. */
       AutoNoJSAPI nojsapi;
 
       mTextEditor->OutputToString(NS_LITERAL_STRING("text/plain"), flags,
                                   aValue);
     }
-    if (canCache) {
+    // Only when the result doesn't include line breaks caused by hard-wrap,
+    // mCacheValue should cache the value.
+    if (!(flags & nsIDocumentEncoder::OutputWrap)) {
       mCachedValue = aValue;
     } else {
       mCachedValue.Truncate();
     }
   } else {
     if (!mTextCtrlElement->ValueChanged() || !mValue) {
       mTextCtrlElement->GetDefaultValueFromContent(aValue);
     } else {
@@ -2714,20 +2712,20 @@ nsTextEditorState::SetValue(const nsAStr
           // the existing selection -- see bug 574558), in which case we don't
           // need to reset the value here.
           if (!mBoundFrame) {
             return SetValue(newValue, aFlags & eSetValue_Notify);
           }
           return true;
         }
 
-        if (!IsSingleLineTextControl()) {
-          if (!mCachedValue.Assign(newValue, fallible)) {
-            return false;
-          }
+        // The new value never includes line breaks caused by hard-wrap.
+        // So, mCachedValue can always cache the new value.
+        if (!mCachedValue.Assign(newValue, fallible)) {
+          return false;
         }
       }
     }
   } else {
     if (!mValue) {
       mValue.emplace();
     }
 

--- a/dom/html/nsTextEditorState.h
+++ b/dom/html/nsTextEditorState.h
@@ -454,17 +454,22 @@ private:
   RefPtr<mozilla::TextEditor> mTextEditor;
   nsCOMPtr<mozilla::dom::Element> mRootNode;
   nsCOMPtr<mozilla::dom::Element> mPlaceholderDiv;
   nsCOMPtr<mozilla::dom::Element> mPreviewDiv;
   nsTextControlFrame* mBoundFrame;
   RefPtr<nsTextInputListener> mTextListener;
   mozilla::Maybe<nsString> mValue;
   RefPtr<nsAnonDivObserver> mMutationObserver;
-  mutable nsString mCachedValue; // Caches non-hard-wrapped value on a multiline control.
+  // Cache of the |.value| of <input> or <textarea> element without hard-wrap.
+  // If this is empty string, it doesn't cache |.value|.
+  // Otherwise, it's cached when setting specific value or getting value from
+  // TextEditor.  Additionally, when contents in the anonymous <div> element
+  // is modified, this is cleared.
+  mutable nsString mCachedValue;
   // mValueBeingSet is available only while SetValue() is requesting to commit
   // composition.  I.e., this is valid only while mIsCommittingComposition is
   // true.  While active composition is being committed, GetValue() needs
   // the latest value which is set by SetValue().  So, this is cache for that.
   nsString mValueBeingSet;
   SelectionProperties mSelectionProperties;
   bool mEverInited; // Have we ever been initialized?
   bool mEditorInitialized;