diff --git a/dom/base/TextInputProcessor.cpp b/dom/base/TextInputProcessor.cpp
index 5d9da4dd9a7..a8365ed9cb3 100644
--- a/dom/base/TextInputProcessor.cpp
+++ b/dom/base/TextInputProcessor.cpp
@@ -596,6 +596,38 @@ TextInputProcessor::Keyup(nsIDOMKeyEvent* aDOMKeyEvent,
   return NS_OK;
 }
 
+NS_IMETHODIMP
+TextInputProcessor::GetModifierState(const nsAString& aModifierKeyName,
+                                     bool* aActive)
+{
+  MOZ_RELEASE_ASSERT(aActive, "aActive must not be null");
+  MOZ_RELEASE_ASSERT(nsContentUtils::IsCallerChrome());
+  if (!mModifierKeyDataArray) {
+    *aActive = false;
+    return NS_OK;
+  }
+  Modifiers activeModifiers = mModifierKeyDataArray->GetActiveModifiers();
+  Modifiers modifier = WidgetInputEvent::GetModifier(aModifierKeyName);
+  *aActive = ((activeModifiers & modifier) != 0);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+TextInputProcessor::ShareModifierStateOf(nsITextInputProcessor* aOther)
+{
+  MOZ_RELEASE_ASSERT(nsContentUtils::IsCallerChrome());
+  if (!aOther) {
+    mModifierKeyDataArray = nullptr;
+    return NS_OK;
+  }
+  TextInputProcessor* other = static_cast<TextInputProcessor*>(aOther);
+  if (!other->mModifierKeyDataArray) {
+    other->mModifierKeyDataArray = new ModifierKeyDataArray();
+  }
+  mModifierKeyDataArray = other->mModifierKeyDataArray;
+  return NS_OK;
+}
+
 /******************************************************************************
  * TextInputProcessor::ModifierKeyData
  ******************************************************************************/
diff --git a/dom/base/TextInputProcessor.h b/dom/base/TextInputProcessor.h
index 003385fd1a9..1544fb5ce1a 100644
--- a/dom/base/TextInputProcessor.h
+++ b/dom/base/TextInputProcessor.h
@@ -8,6 +8,7 @@
 
 #include "mozilla/EventForwards.h"
 #include "mozilla/TextEventDispatcherListener.h"
+#include "nsAutoPtr.h"
 #include "nsITextInputProcessor.h"
 #include "nsITextInputProcessorCallback.h"
 #include "nsTArray.h"
@@ -78,33 +79,51 @@ private:
 
   class ModifierKeyDataArray : public nsTArray<ModifierKeyData>
   {
+    NS_INLINE_DECL_REFCOUNTING(ModifierKeyDataArray)
+
   public:
     Modifiers GetActiveModifiers() const;
     void ActivateModifierKey(const ModifierKeyData& aModifierKeyData);
     void InactivateModifierKey(const ModifierKeyData& aModifierKeyData);
     void ToggleModifierKey(const ModifierKeyData& aModifierKeyData);
+
+  private:
+    virtual ~ModifierKeyDataArray() { }
   };
 
   Modifiers GetActiveModifiers() const
   {
-    return mModifierKeyDataArray.GetActiveModifiers();
+    return mModifierKeyDataArray ?
+      mModifierKeyDataArray->GetActiveModifiers() : 0;
+  }
+  void EnsureModifierKeyDataArray()
+  {
+    if (mModifierKeyDataArray) {
+      return;
+    }
+    mModifierKeyDataArray = new ModifierKeyDataArray();
   }
   void ActivateModifierKey(const ModifierKeyData& aModifierKeyData)
   {
-    mModifierKeyDataArray.ActivateModifierKey(aModifierKeyData);
+    EnsureModifierKeyDataArray();
+    mModifierKeyDataArray->ActivateModifierKey(aModifierKeyData);
   }
   void InactivateModifierKey(const ModifierKeyData& aModifierKeyData)
   {
-    mModifierKeyDataArray.InactivateModifierKey(aModifierKeyData);
+    if (!mModifierKeyDataArray) {
+      return;
+    }
+    mModifierKeyDataArray->InactivateModifierKey(aModifierKeyData);
   }
   void ToggleModifierKey(const ModifierKeyData& aModifierKeyData)
   {
-    mModifierKeyDataArray.ToggleModifierKey(aModifierKeyData);
+    EnsureModifierKeyDataArray();
+    mModifierKeyDataArray->ToggleModifierKey(aModifierKeyData);
   }
 
   TextEventDispatcher* mDispatcher; // [Weak]
   nsCOMPtr<nsITextInputProcessorCallback> mCallback;
-  ModifierKeyDataArray mModifierKeyDataArray;
+  nsRefPtr<ModifierKeyDataArray> mModifierKeyDataArray;
 
   bool mForTests;
 };
diff --git a/dom/base/test/chrome/window_nsITextInputProcessor.xul b/dom/base/test/chrome/window_nsITextInputProcessor.xul
index e4c4a9d6a9c..52a91e587ca 100644
--- a/dom/base/test/chrome/window_nsITextInputProcessor.xul
+++ b/dom/base/test/chrome/window_nsITextInputProcessor.xul
@@ -1155,6 +1155,11 @@ function runKeyTests()
   }
 
   // Modifier state tests
+  var sharedTIP = createTIP();
+  ok(sharedTIP.beginInputTransactionForTests(otherWindow),
+     description + "sharedTIP.beginInputTransactionForTests(otherWindow) should return true");
+  TIP.shareModifierStateOf(sharedTIP);
+  var independentTIP = createTIP();
   const kModifierKeys = [
     { key: "Alt",        code: "AltLeft",      isLockable: false },
     { key: "Alt",        code: "AltRight",     isLockable: false },
@@ -1204,6 +1209,18 @@ function runKeyTests()
     }
   }
 
+  function checkAllTIPModifiers(aTestDesc, aModifiers)
+  {
+    for (var i = 0; i < kModifiers.length; i++) {
+      is(TIP.getModifierState(kModifiers[i]), aModifiers.indexOf(kModifiers[i]) >= 0,
+         aTestDesc + ", TIP.getModifierState(\"" + kModifiers[i] + "\") returns wrong value");
+      is(sharedTIP.getModifierState(kModifiers[i]), TIP.getModifierState(kModifiers[i]),
+         aTestDesc + ", sharedTIP.getModifierState(\"" + kModifiers[i] + "\") returns different value from TIP");
+      is(independentTIP.getModifierState(kModifiers[i]), false,
+         aTestDesc + ", independentTIP.getModifierState(\"" + kModifiers[i] + "\") should return false");
+    }
+  }
+
   // First, all modifiers must be false.
   reset();
   doPreventDefaults = [ "keypress" ];
@@ -1224,9 +1241,13 @@ function runKeyTests()
     var testDesc = "A modifier key \"" + kModifierKeys[i].key + "\" (\"" + kModifierKeys[i].code + "\") and a printable key";
     if (!kModifierKeys[i].isLockable) {
       TIP.keydown(modKey);
+      checkAllTIPModifiers(testDesc + ", \"" + kModifierKeys[i].key + "\" keydown", [ kModifierKeys[i].key ]);
       TIP.keydown(keyA);
+      checkAllTIPModifiers(testDesc + ", \"a\" keydown", [ kModifierKeys[i].key ]);
       TIP.keyup(keyA);
+      checkAllTIPModifiers(testDesc + ", \"a\" keyup", [ kModifierKeys[i].key ]);
       TIP.keyup(modKey);
+      checkAllTIPModifiers(testDesc + ", \"" + kModifierKeys[i].key + "\" keyup", [ ]);
       is(events.length, 5,
          description + testDesc + " should cause 5 events");
       checkModifiers(testDesc, events[0], "keydown",  kModifierKeys[i].key, kModifierKeys[i].code, [ kModifierKeys[i].key ]);
@@ -1236,11 +1257,17 @@ function runKeyTests()
       checkModifiers(testDesc, events[4], "keyup",    kModifierKeys[i].key, kModifierKeys[i].code, [ ]);
     } else {
       TIP.keydown(modKey);
+      checkAllTIPModifiers(testDesc + ", \"" + kModifierKeys[i].key + "\" first keydown", [ kModifierKeys[i].key ]);
       TIP.keyup(modKey);
+      checkAllTIPModifiers(testDesc + ", \"" + kModifierKeys[i].key + "\" first keyup", [ kModifierKeys[i].key ]);
       TIP.keydown(keyA);
+      checkAllTIPModifiers(testDesc + ", \"a\" keydown", [ kModifierKeys[i].key ]);
       TIP.keyup(keyA);
+      checkAllTIPModifiers(testDesc + ", \"a\" keyup", [ kModifierKeys[i].key ]);
       TIP.keydown(modKey);
+      checkAllTIPModifiers(testDesc + ", \"" + kModifierKeys[i].key + "\" second keydown", [ ]);
       TIP.keyup(modKey);
+      checkAllTIPModifiers(testDesc + ", \"" + kModifierKeys[i].key + "\" second keyup", [ ]);
       is(events.length, 7,
          description + testDesc + " should cause 7 events");
       checkModifiers(testDesc, events[0], "keydown",  kModifierKeys[i].key, kModifierKeys[i].code, [ kModifierKeys[i].key ]);
@@ -1258,18 +1285,27 @@ function runKeyTests()
   var shiftRight = new KeyboardEvent("", { key: "Shift", code: "ShiftRight" });
   var shiftVirtual = new KeyboardEvent("", { key: "Shift", code: "" });
   var altGrVirtual = new KeyboardEvent("", { key: "AltGraph", code: "" });
+  var ctrlVirtual = new KeyboardEvent("", { key: "Control", code: "" });
 
   var testDesc = "ShiftLeft press -> ShiftRight press -> ShiftRight release -> ShiftLeft release";
   reset();
   doPreventDefaults = [ "keypress" ];
   TIP.keydown(shiftLeft);
+  checkAllTIPModifiers(testDesc + ", Left-Shift keydown", [ "Shift" ]);
   TIP.keydown(shiftRight);
+  checkAllTIPModifiers(testDesc + ", Right-Shift keydown", [ "Shift" ]);
   TIP.keydown(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keydown", [ "Shift" ]);
   TIP.keyup(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keyup", [ "Shift" ]);
   TIP.keyup(shiftRight);
+  checkAllTIPModifiers(testDesc + ", Right-Shift keyup", [ "Shift" ]);
   TIP.keydown(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keydown (after Right-Shift keyup)", [ "Shift" ]);
   TIP.keyup(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keyup (after Right-Shift keyup)", [ "Shift" ]);
   TIP.keyup(shiftLeft);
+  checkAllTIPModifiers(testDesc + ", Left-Shift keyup", [ ]);
 
   is(events.length, 10,
      description + testDesc + " should cause 10 events");
@@ -1288,13 +1324,21 @@ function runKeyTests()
   reset();
   doPreventDefaults = [ "keypress" ];
   TIP.keydown(shiftLeft);
+  checkAllTIPModifiers(testDesc + ", Left-Shift keydown", [ "Shift" ]);
   TIP.keydown(shiftRight);
+  checkAllTIPModifiers(testDesc + ", Right-Shift keydown", [ "Shift" ]);
   TIP.keydown(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keydown", [ "Shift" ]);
   TIP.keyup(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keyup", [ "Shift" ]);
   TIP.keyup(shiftLeft);
+  checkAllTIPModifiers(testDesc + ", Left-Shift keyup", [ "Shift" ]);
   TIP.keydown(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keydown (after Left-Shift keyup)", [ "Shift" ]);
   TIP.keyup(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keyup (after Left-Shift keyup)", [ "Shift" ]);
   TIP.keyup(shiftRight);
+  checkAllTIPModifiers(testDesc + ", Right-Shift keyup", [ ]);
 
   is(events.length, 10,
      description + testDesc + " should cause 10 events");
@@ -1313,13 +1357,21 @@ function runKeyTests()
   reset();
   doPreventDefaults = [ "keypress" ];
   TIP.keydown(shiftLeft);
+  checkAllTIPModifiers(testDesc + ", Left-Shift keydown", [ "Shift" ]);
   TIP.keydown(shiftVirtual);
+  checkAllTIPModifiers(testDesc + ", Virtual-Shift keydown", [ "Shift" ]);
   TIP.keydown(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keydown", [ "Shift" ]);
   TIP.keyup(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keyup", [ "Shift" ]);
   TIP.keyup(shiftVirtual);
+  checkAllTIPModifiers(testDesc + ", Virtual-Shift keyup", [ "Shift" ]);
   TIP.keydown(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keydown (after Virtual-Shift keyup)", [ "Shift" ]);
   TIP.keyup(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keyup (after Virtual-Shift keyup)", [ "Shift" ]);
   TIP.keyup(shiftLeft);
+  checkAllTIPModifiers(testDesc + ", Left-Shift keyup", [ ]);
 
   is(events.length, 10,
      description + testDesc + " should cause 10 events");
@@ -1338,13 +1390,21 @@ function runKeyTests()
   reset();
   doPreventDefaults = [ "keypress" ];
   TIP.keydown(shiftVirtual);
+  checkAllTIPModifiers(testDesc + ", Virtual-Shift keydown", [ "Shift" ]);
   TIP.keydown(shiftRight);
+  checkAllTIPModifiers(testDesc + ", Right-Shift keydown", [ "Shift" ]);
   TIP.keydown(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keydown", [ "Shift" ]);
   TIP.keyup(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keyup", [ "Shift" ]);
   TIP.keyup(shiftRight);
+  checkAllTIPModifiers(testDesc + ", Right-Shift keyup", [ "Shift" ]);
   TIP.keydown(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keydown (after Right-Shift keyup)", [ "Shift" ]);
   TIP.keyup(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keyup (after Right-Shift keyup)", [ "Shift" ]);
   TIP.keyup(shiftVirtual);
+  checkAllTIPModifiers(testDesc + ", Virtual-Shift keyup", [ ]);
 
   is(events.length, 10,
      description + testDesc + " should cause 10 events");
@@ -1363,16 +1423,27 @@ function runKeyTests()
   reset();
   doPreventDefaults = [ "keypress" ];
   TIP.keydown(shiftLeft);
+  checkAllTIPModifiers(testDesc + ", Left-Shift keydown", [ "Shift" ]);
   TIP.keydown(shiftRight);
+  checkAllTIPModifiers(testDesc + ", Right-Shift keydown", [ "Shift" ]);
   TIP.keydown(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keydown", [ "Shift" ]);
   TIP.keyup(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keyup", [ "Shift" ]);
   TIP.keyup(shiftRight);
+  checkAllTIPModifiers(testDesc + ", Right-Shift keyup", [ "Shift" ]);
   TIP.keydown(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keydown (after Virtual-Shift keyup)", [ "Shift" ]);
   TIP.keyup(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keyup (after Virtual-Shift keyup)", [ "Shift" ]);
   TIP.keyup(shiftRight);
+  checkAllTIPModifiers(testDesc + ", Right-Shift keyup again", [ "Shift" ]);
   TIP.keydown(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keydown (after Virtual-Shift keyup again)", [ "Shift" ]);
   TIP.keyup(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keyup (after Virtual-Shift keyup again)", [ "Shift" ]);
   TIP.keyup(shiftLeft);
+  checkAllTIPModifiers(testDesc + ", Left-Shift keyup", [ ]);
 
   is(events.length, 14,
      description + testDesc + " should cause 14 events");
@@ -1395,15 +1466,25 @@ function runKeyTests()
   reset();
   doPreventDefaults = [ "keypress" ];
   TIP.keydown(shiftLeft);
+  checkAllTIPModifiers(testDesc + ", Left-Shift keydown", [ "Shift" ]);
   TIP.keydown(shiftLeft);
+  checkAllTIPModifiers(testDesc + ", Left-Shift keydown again", [ "Shift" ]);
   TIP.keydown(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keydown", [ "Shift" ]);
   TIP.keyup(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keyup", [ "Shift" ]);
   TIP.keyup(shiftLeft);
+  checkAllTIPModifiers(testDesc + ", Left-Shift keyup", [ ]);
   TIP.keydown(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keydown (after Left-Shift keyup)", [ ]);
   TIP.keyup(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keyup (after Left-Shift keyup)", [ ]);
   TIP.keyup(shiftLeft);
+  checkAllTIPModifiers(testDesc + ", Left-Shift keyup again", [ ]);
   TIP.keydown(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keydown (after Left-Shift keyup again)", [ ]);
   TIP.keyup(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keyup (after Left-Shift keyup again)", [ ]);
 
   is(events.length, 13,
      description + testDesc + " should cause 13 events");
@@ -1425,13 +1506,21 @@ function runKeyTests()
   reset();
   doPreventDefaults = [ "keypress" ];
   TIP.keydown(shiftVirtual);
+  checkAllTIPModifiers(testDesc + ", Virtual-Shift keydown", [ "Shift" ]);
   TIP.keydown(altGrVirtual);
+  checkAllTIPModifiers(testDesc + ", Virtual-AltGraph keydown", [ "Shift", "AltGraph" ]);
   TIP.keydown(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keydown", [ "Shift", "AltGraph" ]);
   TIP.keyup(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keyup", [ "Shift", "AltGraph" ]);
   TIP.keyup(altGrVirtual);
+  checkAllTIPModifiers(testDesc + ", Virtual-AltGraph keyup", [ "Shift" ]);
   TIP.keydown(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keydown (after Virtual-AltGraph keyup)", [ "Shift" ]);
   TIP.keyup(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keyup (after Virtual-AltGraph keyup)", [ "Shift" ]);
   TIP.keyup(shiftVirtual);
+  checkAllTIPModifiers(testDesc + ", Virtual-Shift keyup", [ ]);
 
   is(events.length, 10,
      description + testDesc + " should cause 10 events");
@@ -1450,13 +1539,21 @@ function runKeyTests()
   reset();
   doPreventDefaults = [ "keypress" ];
   TIP.keydown(shiftVirtual);
+  checkAllTIPModifiers(testDesc + ", Virtual-Shift keydown", [ "Shift" ]);
   TIP.keydown(altGrVirtual);
+  checkAllTIPModifiers(testDesc + ", Virtual-AltGraph keydown", [ "Shift", "AltGraph" ]);
   TIP.keydown(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keydown", [ "Shift", "AltGraph" ]);
   TIP.keyup(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keyup", [ "Shift", "AltGraph" ]);
   TIP.keyup(shiftVirtual);
+  checkAllTIPModifiers(testDesc + ", Virtual-Shift keyup", [ "AltGraph" ]);
   TIP.keydown(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keydown (after Virtual-Shift keyup)", [ "AltGraph" ]);
   TIP.keyup(keyA);
+  checkAllTIPModifiers(testDesc + ", \"a\" keyup (after Virtual-Shift keyup)", [ "AltGraph" ]);
   TIP.keyup(altGrVirtual);
+  checkAllTIPModifiers(testDesc + ", Virtual-AltGraph keyup", [ ]);
 
   is(events.length, 10,
      description + testDesc + " should cause 10 events");
@@ -1471,6 +1568,39 @@ function runKeyTests()
   checkModifiers(testDesc, events[8], "keyup",    "a",        "KeyA", [ "AltGraph" ]);
   checkModifiers(testDesc, events[9], "keyup",    "AltGraph", "",     [ ]);
 
+  // shareModifierStateOf(null) should cause resetting the modifier state
+  function checkTIPModifiers(aTestDesc, aTIP, aModifiers)
+  {
+    for (var i = 0; i < kModifiers.length; i++) {
+      is(aTIP.getModifierState(kModifiers[i]), aModifiers.indexOf(kModifiers[i]) >= 0,
+         description + aTestDesc + ", aTIP.getModifierState(\"" + kModifiers[i] + "\") returns wrong value");
+    }
+  }
+  TIP.keydown(shiftVirtual);
+  TIP.keydown(altGrVirtual);
+  sharedTIP.shareModifierStateOf(null);
+  checkTIPModifiers("sharedTIP.sharedModifierStateOf(null) shouldn't cause TIP's modifiers reset", TIP, [ "Shift", "AltGraph" ]);
+  checkTIPModifiers("sharedTIP.sharedModifierStateOf(null) should cause sharedTIP modifiers reset", sharedTIP, [ ]);
+
+  // sharedTIP.shareModifierStateOf(null) should be unlinked from TIP.
+  TIP.keydown(ctrlVirtual);
+  checkTIPModifiers("TIP.keydown(ctrlVirtual) should cause TIP's modifiers set", TIP, [ "Shift", "AltGraph", "Control" ]);
+  checkTIPModifiers("TIP.keydown(ctrlVirtual) shouldn't cause sharedTIP modifiers set", sharedTIP, [ ]);
+
+  // beginInputTransactionForTests() shouldn't cause modifier state reset.
+  ok(TIP.beginInputTransactionForTests(otherWindow),
+     description + "TIP.beginInputTransactionForTests(otherWindow) should return true");
+  checkTIPModifiers("TIP.beginInputTransactionForTests(otherWindow) shouldn't cause TIP's modifiers set", TIP, [ "Shift", "AltGraph", "Control" ]);
+  TIP.keyup(shiftLeft);
+  TIP.keyup(altGrVirtual);
+  TIP.keyup(ctrlVirtual);
+  checkTIPModifiers("TIP should keep modifier's physical key state", TIP, [ "Shift" ]);
+  ok(TIP.beginInputTransactionForTests(window),
+     description + "TIP.beginInputTransactionForTests(window) should return true");
+  checkTIPModifiers("TIP.beginInputTransactionForTests(window) shouldn't cause TIP's modifiers set", TIP, [ "Shift" ]);
+  TIP.keyup(shiftVirtual);
+  checkTIPModifiers("TIP should keep modifier's physical key state", TIP, [ ]);
+
   window.removeEventListener("keydown", handler, false);
   window.removeEventListener("keypress", handler, false);
   window.removeEventListener("keyup", handler, false);
diff --git a/dom/events/UIEvent.cpp b/dom/events/UIEvent.cpp
index 64e833dfce5..9a0dbeeadae 100644
--- a/dom/events/UIEvent.cpp
+++ b/dom/events/UIEvent.cpp
@@ -452,52 +452,7 @@ UIEvent::GetModifierStateInternal(const nsAString& aKey)
 {
   WidgetInputEvent* inputEvent = mEvent->AsInputEvent();
   MOZ_ASSERT(inputEvent, "mEvent must be WidgetInputEvent or derived class");
-  if (aKey.EqualsLiteral("Accel")) {
-    return inputEvent->IsAccel();
-  }
-  if (aKey.EqualsLiteral(NS_DOM_KEYNAME_SHIFT)) {
-    return inputEvent->IsShift();
-  }
-  if (aKey.EqualsLiteral(NS_DOM_KEYNAME_CONTROL)) {
-    return inputEvent->IsControl();
-  }
-  if (aKey.EqualsLiteral(NS_DOM_KEYNAME_META)) {
-    return inputEvent->IsMeta();
-  }
-  if (aKey.EqualsLiteral(NS_DOM_KEYNAME_ALT)) {
-    return inputEvent->IsAlt();
-  }
-
-  if (aKey.EqualsLiteral(NS_DOM_KEYNAME_ALTGRAPH)) {
-    return inputEvent->IsAltGraph();
-  }
-  if (aKey.EqualsLiteral(NS_DOM_KEYNAME_OS)) {
-    return inputEvent->IsOS();
-  }
-
-  if (aKey.EqualsLiteral(NS_DOM_KEYNAME_CAPSLOCK)) {
-    return inputEvent->IsCapsLocked();
-  }
-  if (aKey.EqualsLiteral(NS_DOM_KEYNAME_NUMLOCK)) {
-    return inputEvent->IsNumLocked();
-  }
-
-  if (aKey.EqualsLiteral(NS_DOM_KEYNAME_FN)) {
-    return inputEvent->IsFn();
-  }
-  if (aKey.EqualsLiteral(NS_DOM_KEYNAME_FNLOCK)) {
-    return inputEvent->IsFnLocked();
-  }
-  if (aKey.EqualsLiteral(NS_DOM_KEYNAME_SCROLLLOCK)) {
-    return inputEvent->IsScrollLocked();
-  }
-  if (aKey.EqualsLiteral(NS_DOM_KEYNAME_SYMBOL)) {
-    return inputEvent->IsSymbol();
-  }
-  if (aKey.EqualsLiteral(NS_DOM_KEYNAME_SYMBOLLOCK)) {
-    return inputEvent->IsSymbolLocked();
-  }
-  return false;
+  return ((inputEvent->modifiers & WidgetInputEvent::GetModifier(aKey)) != 0);
 }
 
 } // namespace dom
diff --git a/dom/interfaces/base/nsITextInputProcessor.idl b/dom/interfaces/base/nsITextInputProcessor.idl
index 3b527bd6f61..5fa5f21a93b 100644
--- a/dom/interfaces/base/nsITextInputProcessor.idl
+++ b/dom/interfaces/base/nsITextInputProcessor.idl
@@ -224,7 +224,7 @@ interface nsITextInputProcessorCallback;
  *   TIP.keyup(leftShift);
  */
 
-[scriptable, builtinclass, uuid(16144d6e-a97a-4733-aa6a-3a1287cfe539)]
+[scriptable, builtinclass, uuid(c8b5d3fb-5bed-4b77-b67f-77aee6ac5081)]
 interface nsITextInputProcessor : nsISupports
 {
   /**
@@ -464,6 +464,28 @@ interface nsITextInputProcessor : nsISupports
   [optional_argc]
     boolean keyup(in nsIDOMKeyEvent aKeyboardEvent,
                   [optional] in unsigned long aKeyFlags);
+
+  /**
+   * getModifierState() returns modifier state managed by this instance.
+   *
+   * @param aModifier       One of modifier key names.  This doesn't support
+   *                        virtual modifiers like "Accel".
+   * @return                true if the modifier key is active.  Otherwise,
+   *                        false.
+   */
+  boolean getModifierState(in DOMString aModifierKey);
+
+  /**
+   * shareModifierStateOf() makes the instance shares modifier state of
+   * another instance.  When this is called, the instance refers the modifier
+   * state of another instance.  After that, changes to either this and the
+   * other instance's modifier state is synchronized.
+   *
+   * @param aOther          Another instance which will be referred by the
+   *                        instance.  If this is null, the instance restarts
+   *                        to manage modifier state independently.
+   */
+  void shareModifierStateOf(in nsITextInputProcessor aOther);
 };
 
 %{C++
diff --git a/widget/BasicEvents.h b/widget/BasicEvents.h
index bb8ffdc9f99..237fc8289ee 100644
--- a/widget/BasicEvents.h
+++ b/widget/BasicEvents.h
@@ -1016,6 +1016,11 @@ public:
    */
   static Modifier AccelModifier();
 
+  /**
+   * GetModifier() returns a modifier flag which is activated by aDOMKeyName.
+   */
+  static Modifier GetModifier(const nsAString& aDOMKeyName);
+
   // true indicates the accel key on the environment is down
   bool IsAccel() const
   {
diff --git a/widget/WidgetEventImpl.cpp b/widget/WidgetEventImpl.cpp
index 19201a2abf0..bb1fe031f2f 100644
--- a/widget/WidgetEventImpl.cpp
+++ b/widget/WidgetEventImpl.cpp
@@ -254,6 +254,17 @@ WidgetEvent::IsAllowedToDispatchDOMEvent() const
  * mozilla::WidgetInputEvent
  ******************************************************************************/
 
+/* static */
+Modifier
+WidgetInputEvent::GetModifier(const nsAString& aDOMKeyName)
+{
+  if (aDOMKeyName.EqualsLiteral("Accel")) {
+    return AccelModifier();
+  }
+  KeyNameIndex keyNameIndex = WidgetKeyboardEvent::GetKeyNameIndex(aDOMKeyName);
+  return WidgetKeyboardEvent::GetModifierForKeyName(keyNameIndex);
+}
+
 /* static */
 Modifier
 WidgetInputEvent::AccelModifier()