--- a/widget/TextEventDispatcher.cpp
+++ b/widget/TextEventDispatcher.cpp
@@ -593,39 +593,43 @@ TextEventDispatcher::PendingComposition:
 }
 
 void
 TextEventDispatcher::PendingComposition::Clear()
 {
   mString.Truncate();
   mClauses = nullptr;
   mCaret.mRangeType = TextRangeType::eUninitialized;
+  mReplacedNativeLineBreakers = false;
 }
 
 void
 TextEventDispatcher::PendingComposition::EnsureClauseArray()
 {
   if (mClauses) {
     return;
   }
   mClauses = new TextRangeArray();
 }
 
 nsresult
 TextEventDispatcher::PendingComposition::SetString(const nsAString& aString)
 {
+  MOZ_ASSERT(!mReplacedNativeLineBreakers);
   mString = aString;
   return NS_OK;
 }
 
 nsresult
 TextEventDispatcher::PendingComposition::AppendClause(
                                            uint32_t aLength,
                                            TextRangeType aTextRangeType)
 {
+  MOZ_ASSERT(!mReplacedNativeLineBreakers);
+
   if (NS_WARN_IF(!aLength)) {
     return NS_ERROR_INVALID_ARG;
   }
 
   switch (aTextRangeType) {
     case TextRangeType::eRawClause:
     case TextRangeType::eSelectedRawClause:
     case TextRangeType::eConvertedClause:
@@ -643,16 +647,18 @@ TextEventDispatcher::PendingComposition:
       return NS_ERROR_INVALID_ARG;
   }
 }
 
 nsresult
 TextEventDispatcher::PendingComposition::SetCaret(uint32_t aOffset,
                                                   uint32_t aLength)
 {
+  MOZ_ASSERT(!mReplacedNativeLineBreakers);
+
   mCaret.mStartOffset = aOffset;
   mCaret.mEndOffset = mCaret.mStartOffset + aLength;
   mCaret.mRangeType = TextRangeType::eCaret;
   return NS_OK;
 }
 
 nsresult
 TextEventDispatcher::PendingComposition::Set(const nsAString& aString,
@@ -689,16 +695,18 @@ TextEventDispatcher::PendingComposition:
   }
   ReplaceNativeLineBreakers();
   return NS_OK;
 }
 
 void
 TextEventDispatcher::PendingComposition::ReplaceNativeLineBreakers()
 {
+  mReplacedNativeLineBreakers = true;
+
   // If the composition string is empty, we don't need to do anything.
   if (mString.IsEmpty()) {
     return;
   }
 
   nsAutoString nativeString(mString);
   // Don't expose CRLF to web contents, instead, use LF.
   mString.ReplaceSubstring(NS_LITERAL_STRING("\r\n"), NS_LITERAL_STRING("\n"));
@@ -774,16 +782,22 @@ TextEventDispatcher::PendingComposition:
       NS_WARNING("Caret position is out of the composition string");
       Clear();
       return NS_ERROR_ILLEGAL_VALUE;
     }
     EnsureClauseArray();
     mClauses->AppendElement(mCaret);
   }
 
+  // If the composition string is set without Set(), we need to replace native
+  // line breakers in the composition string with XP line breaker.
+  if (!mReplacedNativeLineBreakers) {
+    ReplaceNativeLineBreakers();
+  }
+
   RefPtr<TextEventDispatcher> kungFuDeathGrip(aDispatcher);
   nsCOMPtr<nsIWidget> widget(aDispatcher->mWidget);
   WidgetCompositionEvent compChangeEvent(true, eCompositionChange, widget);
   aDispatcher->InitEvent(compChangeEvent);
   if (aEventTime) {
     compChangeEvent.AssignEventTime(*aEventTime);
   }
   compChangeEvent.mData = mString;

--- a/widget/TextEventDispatcher.h
+++ b/widget/TextEventDispatcher.h
@@ -325,16 +325,17 @@ private:
                    const WidgetEventTime* aEventTime);
     const TextRangeArray* GetClauses() const { return mClauses; }
     void Clear();
 
   private:
     nsString mString;
     RefPtr<TextRangeArray> mClauses;
     TextRange mCaret;
+    bool mReplacedNativeLineBreakers;
 
     void EnsureClauseArray();
 
     /**
      * ReplaceNativeLineBreakers() replaces "\r\n" and "\r" to "\n" and adjust
      * each clause information and the caret information.
      */
     void ReplaceNativeLineBreakers();