--- a/dom/events/TextComposition.cpp
+++ b/dom/events/TextComposition.cpp
@@ -220,17 +220,17 @@ RemoveControlCharactersFrom(nsAString& a
   if (NS_WARN_IF(!dest)) {
     return;
   }
 
   char16_t* curDest = dest + firstControlCharOffset;
   size_t i = firstControlCharOffset;
   for (const char16_t* source = sourceBegin + firstControlCharOffset;
        source < sourceEnd; ++source) {
-    if (*source == '\t' || !IsControlChar(*source)) {
+    if (*source == '\t' || *source == '\n' || !IsControlChar(*source)) {
       *curDest = *source;
       ++curDest;
       ++i;
     } else if (aRanges) {
       aRanges->RemoveCharacter(i);
     }
   }
 

--- a/widget/TextEventDispatcher.cpp
+++ b/widget/TextEventDispatcher.cpp
@@ -314,19 +314,21 @@ TextEventDispatcher::CommitComposition(n
                                          eCompositionCommitAsIs;
   WidgetCompositionEvent compositionCommitEvent(true, message, widget);
   InitEvent(compositionCommitEvent);
   if (aEventTime) {
     compositionCommitEvent.AssignEventTime(*aEventTime);
   }
   if (message == eCompositionCommit) {
     compositionCommitEvent.mData = *aCommitString;
-    // Don't send CRLF, replace it with LF here.
+    // Don't send CRLF nor CR, replace it with LF here.
     compositionCommitEvent.mData.ReplaceSubstring(NS_LITERAL_STRING("\r\n"),
                                                   NS_LITERAL_STRING("\n"));
+    compositionCommitEvent.mData.ReplaceSubstring(NS_LITERAL_STRING("\r"),
+                                                  NS_LITERAL_STRING("\n"));
   }
   rv = DispatchEvent(widget, compositionCommitEvent, aStatus);
   if (NS_WARN_IF(NS_FAILED(rv))) {
     return rv;
   }
 
   return NS_OK;
 }
@@ -703,18 +705,19 @@ TextEventDispatcher::PendingComposition:
   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.
+  // Don't expose CRLF nor CR to web contents, instead, use LF.
   mString.ReplaceSubstring(NS_LITERAL_STRING("\r\n"), NS_LITERAL_STRING("\n"));
+  mString.ReplaceSubstring(NS_LITERAL_STRING("\r"), NS_LITERAL_STRING("\n"));
 
   // If the length isn't changed, we don't need to adjust any offset and length
   // of mClauses nor mCaret.
   if (nativeString.Length() == mString.Length()) {
     return;
   }
 
   if (mClauses) {

--- a/widget/tests/window_composition_text_querycontent.xul
+++ b/widget/tests/window_composition_text_querycontent.xul
@@ -6001,18 +6001,16 @@ function runNativeLineBreakerTest()
   }
 
   SpecialPowers.setBoolPref("dom.compositionevent.allow_control_characters", false);
 
   textarea.addEventListener("compositionupdate", handler, true);
   textarea.addEventListener("compositionend", handler, true);
 
   // '\n' in composition string shouldn't be changed.
-  // XXX Currently, \n isn't accepted by TextComposition.  So,these test checks
-  //     without the length of \n (bug 1339331).
   clearResult();
   textarea.value = "";
   var clauses = [ "abc\n", "def\n\ng", "hi\n", "\njkl" ];
   var caret = clauses[0] + clauses[1] + clauses[2];
   synthesizeCompositionChange(
     { "composition":
       { "string": clauses.join(''),
         "clauses":
@@ -6025,28 +6023,28 @@ function runNativeLineBreakerTest()
             "attr": COMPOSITION_ATTR_CONVERTED_CLAUSE },
           { "length": clauses[3].length,
             "attr": COMPOSITION_ATTR_SELECTED_CLAUSE },
         ]
       },
       "caret": { "start": caret.length, "length": 0 }
     });
 
-  checkSelection(caret.replace(/\n/g, "").length, "", "runNativeLineBreakerTest", "#1");
-  checkIMESelection("RawClause", true, 0, clauses[0].replace(/\n/g, ""), "runNativeLineBreakerTest: \\n shouldn't be replaced with any character #1");
-  checkIMESelection("SelectedRawClause", true, clauses[0].replace(/\n/g, "").length, clauses[1].replace(/\n/g, ""), "runNativeLineBreakerTest: \\n shouldn't be replaced with any character #1");
-  checkIMESelection("ConvertedClause", true, (clauses[0] + clauses[1]).replace(/\n/g, "").length, clauses[2].replace(/\n/g, ""), "runNativeLineBreakerTest: \\n shouldn't be replaced with any character #1");
-  checkIMESelection("SelectedClause", true, (clauses[0] + clauses[1] + clauses[2]).replace(/\n/g, "").length, clauses[3].replace(/\n/g, ""), "runNativeLineBreakerTest: \\n shouldn't be replaced with any character #1");
-  is(result.compositionupdate, clauses.join('').replace(/\n/g, ""), "runNativeLineBreakerTest: \\n in compositionupdate.data shouldn't be removed nor replaced with other characters #1");
-  is(textarea.value, clauses.join('').replace(/\n/g, ""), "runNativeLineBreakerTest: \\n in textarea.value shouldn't be removed nor replaced with other characters #1");
+  checkSelection(caret.replace(/\n/g, "\n").length + (kLFLen - 1) * 4, "", "runNativeLineBreakerTest", "#1");
+  checkIMESelection("RawClause", true, 0, clauses[0].replace(/\n/g, kLF), "runNativeLineBreakerTest: \\n shouldn't be replaced with any character #1");
+  checkIMESelection("SelectedRawClause", true, clauses[0].replace(/\n/g, kLF).length, clauses[1].replace(/\n/g, kLF), "runNativeLineBreakerTest: \\n shouldn't be replaced with any character #1");
+  checkIMESelection("ConvertedClause", true, (clauses[0] + clauses[1]).replace(/\n/g, kLF).length, clauses[2].replace(/\n/g, kLF), "runNativeLineBreakerTest: \\n shouldn't be replaced with any character #1");
+  checkIMESelection("SelectedClause", true, (clauses[0] + clauses[1] + clauses[2]).replace(/\n/g, kLF).length, clauses[3].replace(/\n/g, kLF), "runNativeLineBreakerTest: \\n shouldn't be replaced with any character #1");
+  is(result.compositionupdate, clauses.join('').replace(/\n/g, "\n"), "runNativeLineBreakerTest: \\n in compositionupdate.data shouldn't be removed nor replaced with other characters #1");
+  is(textarea.value, clauses.join('').replace(/\n/g, "\n"), "runNativeLineBreakerTest: \\n in textarea.value shouldn't be removed nor replaced with other characters #1");
 
   synthesizeComposition({ type: "compositioncommit", data: clauses.join('') });
-  checkSelection(clauses.join('').replace(/\n/g, "").length, "", "runNativeLineBreakerTest", "#2");
-  is(result.compositionend, clauses.join('').replace(/\n/g, ""), "runNativeLineBreakerTest: \\n in compositionend.data shouldn't be removed nor replaced with other characters #2");
-  is(textarea.value, clauses.join('').replace(/\n/g, ""), "runNativeLineBreakerTest: \\n in textarea.value shouldn't be removed nor replaced with other characters #2");
+  checkSelection(clauses.join('').replace(/\n/g, "\n").length + (kLFLen - 1) * 5, "", "runNativeLineBreakerTest", "#2");
+  is(result.compositionend, clauses.join('').replace(/\n/g, "\n"), "runNativeLineBreakerTest: \\n in compositionend.data shouldn't be removed nor replaced with other characters #2");
+  is(textarea.value, clauses.join('').replace(/\n/g, "\n"), "runNativeLineBreakerTest: \\n in textarea.value shouldn't be removed nor replaced with other characters #2");
 
   // \r\n in composition string should be replaced with \n.
   clearResult();
   textarea.value = "";
   clauses = [ "abc\r\n", "def\r\n\r\ng", "hi\r\n", "\r\njkl" ];
   caret = clauses[0] + clauses[1] + clauses[2];
   synthesizeCompositionChange(
     { "composition":
@@ -6061,28 +6059,64 @@ function runNativeLineBreakerTest()
             "attr": COMPOSITION_ATTR_CONVERTED_CLAUSE },
           { "length": clauses[3].length,
             "attr": COMPOSITION_ATTR_SELECTED_CLAUSE },
         ]
       },
       "caret": { "start": caret.length, "length": 0 }
     });
 
-  checkSelection(caret.replace(/\r\n/g, "").length, "", "runNativeLineBreakerTest", "#3");
-  checkIMESelection("RawClause", true, 0, clauses[0].replace(/\r\n/g, ""), "runNativeLineBreakerTest: \\r\\n should be replaced with \\n #3");
-  checkIMESelection("SelectedRawClause", true, clauses[0].replace(/\r\n/g, "").length, clauses[1].replace(/\r\n/g, ""), "runNativeLineBreakerTest: \\r\\n should be replaced with \\n #3");
-  checkIMESelection("ConvertedClause", true, (clauses[0] + clauses[1]).replace(/\r\n/g, "").length, clauses[2].replace(/\r\n/g, ""), "runNativeLineBreakerTest: \\r\\n should be replaced with \\n #3");
-  checkIMESelection("SelectedClause", true, (clauses[0] + clauses[1] + clauses[2]).replace(/\r\n/g, "").length, clauses[3].replace(/\r\n/g, ""), "runNativeLineBreakerTest: \\r\\n should be replaced with \\n #3");
-  is(result.compositionupdate, clauses.join('').replace(/\r\n/g, ""), "runNativeLineBreakerTest: \\r\\n in compositionudpate.data should be replaced with \\n #3");
-  is(textarea.value, clauses.join('').replace(/\r\n/g, ""), "runNativeLineBreakerTest: \\r\\n in textarea.value should be replaced with \\n #3");
+  checkSelection(caret.replace(/\r\n/g, "\n").length + (kLFLen - 1) * 4, "", "runNativeLineBreakerTest", "#3");
+  checkIMESelection("RawClause", true, 0, clauses[0].replace(/\r\n/g, kLF), "runNativeLineBreakerTest: \\r\\n should be replaced with \\n #3");
+  checkIMESelection("SelectedRawClause", true, clauses[0].replace(/\r\n/g, kLF).length, clauses[1].replace(/\r\n/g, kLF), "runNativeLineBreakerTest: \\r\\n should be replaced with \\n #3");
+  checkIMESelection("ConvertedClause", true, (clauses[0] + clauses[1]).replace(/\r\n/g, kLF).length, clauses[2].replace(/\r\n/g, kLF), "runNativeLineBreakerTest: \\r\\n should be replaced with \\n #3");
+  checkIMESelection("SelectedClause", true, (clauses[0] + clauses[1] + clauses[2]).replace(/\r\n/g, kLF).length, clauses[3].replace(/\r\n/g, kLF), "runNativeLineBreakerTest: \\r\\n should be replaced with \\n #3");
+  is(result.compositionupdate, clauses.join('').replace(/\r\n/g, "\n"), "runNativeLineBreakerTest: \\r\\n in compositionudpate.data should be replaced with \\n #3");
+  is(textarea.value, clauses.join('').replace(/\r\n/g, "\n"), "runNativeLineBreakerTest: \\r\\n in textarea.value should be replaced with \\n #3");
 
   synthesizeComposition({ type: "compositioncommit", data: clauses.join('') });
-  checkSelection(clauses.join('').replace(/\r\n/g, "").length, "", "runNativeLineBreakerTest", "#4");
-  is(result.compositionend, clauses.join('').replace(/\r\n/g, ""), "runNativeLineBreakerTest: \\r\\n in compositionend.data should be replaced with \\n #4");
-  is(textarea.value, clauses.join('').replace(/\r\n/g, ""), "runNativeLineBreakerTest: \\r\\n in textarea.value should be replaced with \\n #4");
+  checkSelection(clauses.join('').replace(/\r\n/g, "\n").length + (kLFLen - 1) * 5, "", "runNativeLineBreakerTest", "#4");
+  is(result.compositionend, clauses.join('').replace(/\r\n/g, "\n"), "runNativeLineBreakerTest: \\r\\n in compositionend.data should be replaced with \\n #4");
+  is(textarea.value, clauses.join('').replace(/\r\n/g, "\n"), "runNativeLineBreakerTest: \\r\\n in textarea.value should be replaced with \\n #4");
+
+  // \r (not followed by \n) in composition string should be replaced with \n.
+  clearResult();
+  textarea.value = "";
+  clauses = [ "abc\r", "def\r\rg", "hi\r", "\rjkl" ];
+  caret = clauses[0] + clauses[1] + clauses[2];
+  synthesizeCompositionChange(
+    { "composition":
+      { "string": clauses.join(''),
+        "clauses":
+        [
+          { "length": clauses[0].length,
+            "attr": COMPOSITION_ATTR_RAW_CLAUSE },
+          { "length": clauses[1].length,
+            "attr": COMPOSITION_ATTR_SELECTED_RAW_CLAUSE },
+          { "length": clauses[2].length,
+            "attr": COMPOSITION_ATTR_CONVERTED_CLAUSE },
+          { "length": clauses[3].length,
+            "attr": COMPOSITION_ATTR_SELECTED_CLAUSE },
+        ]
+      },
+      "caret": { "start": caret.length, "length": 0 }
+    });
+
+  checkSelection(caret.replace(/\r/g, "\n").length + (kLFLen - 1) * 4, "", "runNativeLineBreakerTest", "#5");
+  checkIMESelection("RawClause", true, 0, clauses[0].replace(/\r/g, kLF), "runNativeLineBreakerTest: \\r should be replaced with \\n #5");
+  checkIMESelection("SelectedRawClause", true, clauses[0].replace(/\r/g, kLF).length, clauses[1].replace(/\r/g, kLF), "runNativeLineBreakerTest: \\r should be replaced with \\n #5");
+  checkIMESelection("ConvertedClause", true, (clauses[0] + clauses[1]).replace(/\r/g, kLF).length, clauses[2].replace(/\r/g, kLF), "runNativeLineBreakerTest: \\r should be replaced with \\n #5");
+  checkIMESelection("SelectedClause", true, (clauses[0] + clauses[1] + clauses[2]).replace(/\r/g, kLF).length, clauses[3].replace(/\r/g, kLF), "runNativeLineBreakerTest: \\r should be replaced with \\n #5");
+  is(result.compositionupdate, clauses.join('').replace(/\r/g, "\n"), "runNativeLineBreakerTest: \\r in compositionupdate.data should be replaced with \\n #5");
+  is(textarea.value, clauses.join('').replace(/\r/g, "\n"), "runNativeLineBreakerTest: \\r in textarea.value should be replaced with \\n #5");
+
+  synthesizeComposition({ type: "compositioncommit", data: clauses.join('') });
+  checkSelection(clauses.join('').replace(/\r/g, "\n").length + (kLFLen - 1) * 5, "", "runNativeLineBreakerTest", "#6");
+  is(result.compositionend, clauses.join('').replace(/\r/g, "\n"), "runNativeLineBreakerTest: \\r in compositionend.data should be replaced with \\n #6");
+  is(textarea.value, clauses.join('').replace(/\r/g, "\n"), "runNativeLineBreakerTest: \\r in textarea.value should be replaced with \\n #6");
 
   textarea.removeEventListener("compositionupdate", handler, true);
   textarea.removeEventListener("compositionend", handler, true);
 
   SpecialPowers.clearUserPref("dom.compositionevent.allow_control_characters");
 }
 
 function runControlCharTest()
@@ -6101,17 +6135,17 @@ function runControlCharTest()
   }
 
   textarea.addEventListener("compositionupdate", handler, true);
   textarea.addEventListener("compositionend", handler, true);
 
   textarea.value = "";
 
   var controlChars = String.fromCharCode.apply(null, Object.keys(Array.from({length:0x20}))) + "\x7F";
-  var allowedChars = "\t";
+  var allowedChars = "\t\n\n";
 
   var data = "AB" + controlChars + "CD" + controlChars + "EF";
   var removedData = "AB" + allowedChars + "CD" + allowedChars + "EF";
 
   var DIndex = data.indexOf("D");
   var removedDIndex = removedData.indexOf("D");
 
   // input string contains control characters
@@ -6125,17 +6159,17 @@ function runControlCharTest()
             "attr": COMPOSITION_ATTR_SELECTED_CLAUSE },
           { "length": data.length - DIndex,
             "attr": COMPOSITION_ATTR_CONVERTED_CLAUSE }
         ]
       },
       "caret": { "start": DIndex, "length": 0 }
     });
 
-  checkSelection(removedDIndex, "", "runControlCharTest", "#1")
+  checkSelection(removedDIndex + (kLFLen - 1) * 2, "", "runControlCharTest", "#1")
 
   is(result.compositionupdate, removedData, "runControlCharTest: control characters in event.data should be removed in compositionupdate event #1");
   is(textarea.value, removedData, "runControlCharTest: control characters should not appear in textarea #1");
 
   synthesizeComposition({ type: "compositioncommit", data: data });
 
   is(result.compositionend, removedData, "runControlCharTest: control characters in event.data should be removed in compositionend event #2");
   is(textarea.value, removedData, "runControlCharTest: control characters should not appear in textarea #2");
@@ -6157,24 +6191,24 @@ function runControlCharTest()
             "attr": COMPOSITION_ATTR_SELECTED_CLAUSE },
           { "length": data.length - DIndex,
             "attr": COMPOSITION_ATTR_CONVERTED_CLAUSE }
         ]
       },
       "caret": { "start": DIndex, "length": 0 }
     });
 
-  checkSelection(DIndex - 1 + kLFLen, "", "runControlCharTest", "#3")
-
-  is(result.compositionupdate, data, "runControlCharTest: control characters in event.data should not be removed in compositionupdate event #3");
+  checkSelection(DIndex + (kLFLen - 1) * 2, "", "runControlCharTest", "#3")
+
+  is(result.compositionupdate, data.replace(/\r/g, "\n"), "runControlCharTest: control characters in event.data should not be removed in compositionupdate event #3");
   is(textarea.value, data.replace(/\r/g, "\n"), "runControlCharTest: control characters should appear in textarea #3");
 
   synthesizeComposition({ type: "compositioncommit", data: data });
 
-  is(result.compositionend, data, "runControlCharTest: control characters in event.data should not be removed in compositionend event #4");
+  is(result.compositionend, data.replace(/\r/g, "\n"), "runControlCharTest: control characters in event.data should not be removed in compositionend event #4");
   is(textarea.value, data.replace(/\r/g, "\n"), "runControlCharTest: control characters should appear in textarea #4");
 
   SpecialPowers.clearUserPref("dom.compositionevent.allow_control_characters");
 
   textarea.removeEventListener("compositionupdate", handler, true);
   textarea.removeEventListener("compositionend", handler, true);
 }