--- a/widget/windows/KeyboardLayout.cpp
+++ b/widget/windows/KeyboardLayout.cpp
@@ -3654,27 +3654,27 @@ KeyboardLayout::InitNativeKey(NativeKey&
   MOZ_ASSERT(aNativeKey.mKeyNameIndex == KEY_NAME_INDEX_USE_STRING,
     "Printable key's key name index must be KEY_NAME_INDEX_USE_STRING");
 
   bool isKeyDown = aNativeKey.IsKeyDownMessage();
 
   if (IsDeadKey(virtualKey, aModKeyState)) {
     if ((isKeyDown && mActiveDeadKey < 0) ||
         (!isKeyDown && mActiveDeadKey == virtualKey)) {
-      //  First dead key event doesn't generate characters.
-      if (isKeyDown) {
-        // Dead-key state activated at keydown.
-        mActiveDeadKey = virtualKey;
-        mDeadKeyShiftState =
-          VirtualKey::ModifierKeyStateToShiftState(aModKeyState);
-      }
+      ActivateDeadKeyState(aNativeKey, aModKeyState);
+#ifdef DEBUG
       UniCharsAndModifiers deadChars =
         GetNativeUniCharsAndModifiers(virtualKey, aModKeyState);
-      NS_ASSERTION(deadChars.mLength == 1,
-                   "dead key must generate only one character");
+      MOZ_ASSERT(deadChars.mLength == 1,
+                 "dead key must generate only one character");
+#endif
+      // First dead key event doesn't generate characters.  Dead key should
+      // cause only keydown event and keyup event whose KeyboardEvent.key
+      // values are "Dead".
+      aNativeKey.mCommittedCharsAndModifiers.Clear();
       aNativeKey.mKeyNameIndex = KEY_NAME_INDEX_Dead;
       return;
     }
 
     // At keydown message handling, we need to forget the first dead key
     // because there is no guarantee coming WM_KEYUP for the second dead
     // key before next WM_KEYDOWN.  E.g., due to auto key repeat or pressing
     // another dead key before releasing current key.  Therefore, we can
@@ -4073,16 +4073,31 @@ KeyboardLayout::EnsureDeadKeyActive(bool
     // >1 - Previous pressed key does not produce any composite characters.
     //      Return dead-key character followed by base character(s).
   } while ((ret < 0) != aIsActive);
 
   return (ret < 0);
 }
 
 void
+KeyboardLayout::ActivateDeadKeyState(const NativeKey& aNativeKey,
+                                     const ModifierKeyState& aModKeyState)
+{
+  // Dead-key state should be activated at keydown.
+  if (!aNativeKey.IsKeyDownMessage()) {
+    return;
+  }
+
+  MOZ_RELEASE_ASSERT(IsPrintableCharKey(aNativeKey.mOriginalVirtualKeyCode));
+
+  mActiveDeadKey = aNativeKey.mOriginalVirtualKeyCode;
+  mDeadKeyShiftState = VirtualKey::ModifierKeyStateToShiftState(aModKeyState);
+}
+
+void
 KeyboardLayout::DeactivateDeadKeyState()
 {
   if (mActiveDeadKey < 0) {
     return;
   }
 
   BYTE kbdState[256];
   memset(kbdState, 0, sizeof(kbdState));

--- a/widget/windows/KeyboardLayout.h
+++ b/widget/windows/KeyboardLayout.h
@@ -636,17 +636,23 @@ private:
                                 DeadKeyEntry* aDeadKeyArray, uint32_t aEntries);
   bool EnsureDeadKeyActive(bool aIsActive, uint8_t aDeadKey,
                              const PBYTE aDeadKeyKbdState);
   uint32_t GetDeadKeyCombinations(uint8_t aDeadKey,
                                   const PBYTE aDeadKeyKbdState,
                                   uint16_t aShiftStatesWithBaseChars,
                                   DeadKeyEntry* aDeadKeyArray,
                                   uint32_t aMaxEntries);
+  /**
+   * Activates or deactivates dead key state.
+   */
+  void ActivateDeadKeyState(const NativeKey& aNativeKey,
+                            const ModifierKeyState& aModKeyState);
   void DeactivateDeadKeyState();
+
   const DeadKeyTable* AddDeadKeyTable(const DeadKeyEntry* aDeadKeyArray,
                                       uint32_t aEntries);
   void ReleaseDeadKeyTables();
 
   /**
    * Loads the specified keyboard layout. This method always clear the dead key
    * state.
    */