--- a/widget/windows/TSFTextStore.cpp
+++ b/widget/windows/TSFTextStore.cpp
@@ -5198,17 +5198,22 @@ TSFTextStore::RecordCompositionEndAction
   MaybeDispatchKeyboardEventAsProcessedByIME();
   if (mDestroyed) {
     MOZ_LOG(sTextStoreLog, LogLevel::Error,
       ("0x%p   TSFTextStore::RecordCompositionEndAction() FAILED due to "
        "destroyed during dispatching a keyboard event", this));
     return false;
   }
 
-  CompleteLastActionIfStillIncomplete();
+  // If we're handling incomplete composition update or already handled
+  // composition update, we can forget them since composition end will send
+  // the latest composition string and it overwrites the composition string
+  // even if we dispatch eCompositionChange event before that.  So, let's
+  // forget all composition updates now.
+  RemoveLastCompositionUpdateActions();
   PendingAction* action = mPendingActions.AppendElement();
   action->mType = PendingAction::Type::eCompositionEnd;
   action->mData = mComposition.mString;
 
   Content& contentForTSF = ContentForTSFRef();
   if (!contentForTSF.IsInitialized()) {
     MOZ_LOG(sTextStoreLog, LogLevel::Error,
       ("0x%p   TSFTextStore::RecordCompositionEndAction() FAILED due "

--- a/widget/windows/TSFTextStore.h
+++ b/widget/windows/TSFTextStore.h
@@ -777,16 +777,27 @@ protected:
   void CompleteLastActionIfStillIncomplete()
   {
     if (!IsPendingCompositionUpdateIncomplete()) {
       return;
     }
     RecordCompositionUpdateAction();
   }
 
+  void RemoveLastCompositionUpdateActions()
+  {
+    while (!mPendingActions.IsEmpty()) {
+      const PendingAction& lastAction = mPendingActions.LastElement();
+      if (lastAction.mType != PendingAction::Type::eCompositionUpdate) {
+        break;
+      }
+      mPendingActions.RemoveLastElement();
+    }
+  }
+
   // When On*Composition() is called without document lock, we need to flush
   // the recorded actions at quitting the method.
   // AutoPendingActionAndContentFlusher class is usedful for it.
   class MOZ_STACK_CLASS AutoPendingActionAndContentFlusher final
   {
   public:
     explicit AutoPendingActionAndContentFlusher(TSFTextStore* aTextStore)
       : mTextStore(aTextStore)