diff --git a/content/events/src/TextComposition.cpp b/content/events/src/TextComposition.cpp
index cabfab625f42..bf75f36059ef 100644
--- a/content/events/src/TextComposition.cpp
+++ b/content/events/src/TextComposition.cpp
@@ -25,8 +25,7 @@ TextComposition::TextComposition(nsPresContext* aPresContext,
                                  nsGUIEvent* aEvent) :
   mPresContext(aPresContext), mNode(aNode),
   mNativeContext(aEvent->widget->GetInputContext().mNativeIMEContext),
-  mIsSynthesizedForTests(
-    (aEvent->flags & NS_EVENT_FLAG_SYNTHETIC_TEST_EVENT) != 0)
+  mIsSynthesizedForTests(aEvent->mFlags.mIsSynthesizedForTests)
 {
 }
 
diff --git a/content/events/src/nsIMEStateManager.cpp b/content/events/src/nsIMEStateManager.cpp
index 69a957ab6c3b..75aa6b148fd0 100644
--- a/content/events/src/nsIMEStateManager.cpp
+++ b/content/events/src/nsIMEStateManager.cpp
@@ -624,7 +624,7 @@ nsIMEStateManager::NotifyIME(NotificationToIME aNotification,
       if (!backup.GetLastData().IsEmpty()) {
         nsTextEvent textEvent(true, NS_TEXT_TEXT, widget);
         textEvent.theText = backup.GetLastData();
-        textEvent.flags |= NS_EVENT_FLAG_SYNTHETIC_TEST_EVENT;
+        textEvent.mFlags.mIsSynthesizedForTests = true;
         widget->DispatchEvent(&textEvent, status);
         if (widget->Destroyed()) {
           return NS_OK;
@@ -634,7 +634,7 @@ nsIMEStateManager::NotifyIME(NotificationToIME aNotification,
       status = nsEventStatus_eIgnore;
       nsCompositionEvent endEvent(true, NS_COMPOSITION_END, widget);
       endEvent.data = backup.GetLastData();
-      endEvent.flags |= NS_EVENT_FLAG_SYNTHETIC_TEST_EVENT;
+      endEvent.mFlags.mIsSynthesizedForTests = true;
       widget->DispatchEvent(&endEvent, status);
 
       return NS_OK;
@@ -647,7 +647,7 @@ nsIMEStateManager::NotifyIME(NotificationToIME aNotification,
       if (!backup.GetLastData().IsEmpty()) {
         nsCompositionEvent updateEvent(true, NS_COMPOSITION_UPDATE, widget);
         updateEvent.data = backup.GetLastData();
-        updateEvent.flags |= NS_EVENT_FLAG_SYNTHETIC_TEST_EVENT;
+        updateEvent.mFlags.mIsSynthesizedForTests = true;
         widget->DispatchEvent(&updateEvent, status);
         if (widget->Destroyed()) {
           return NS_OK;
@@ -656,7 +656,7 @@ nsIMEStateManager::NotifyIME(NotificationToIME aNotification,
         status = nsEventStatus_eIgnore;
         nsTextEvent textEvent(true, NS_TEXT_TEXT, widget);
         textEvent.theText = backup.GetLastData();
-        textEvent.flags |= NS_EVENT_FLAG_SYNTHETIC_TEST_EVENT;
+        textEvent.mFlags.mIsSynthesizedForTests = true;
         widget->DispatchEvent(&textEvent, status);
         if (widget->Destroyed()) {
           return NS_OK;
@@ -666,7 +666,7 @@ nsIMEStateManager::NotifyIME(NotificationToIME aNotification,
       status = nsEventStatus_eIgnore;
       nsCompositionEvent endEvent(true, NS_COMPOSITION_END, widget);
       endEvent.data = backup.GetLastData();
-      endEvent.flags |= NS_EVENT_FLAG_SYNTHETIC_TEST_EVENT;
+      endEvent.mFlags.mIsSynthesizedForTests = true;
       widget->DispatchEvent(&endEvent, status);
 
       return NS_OK;
diff --git a/dom/base/nsDOMWindowUtils.cpp b/dom/base/nsDOMWindowUtils.cpp
index f9975376c6b4..db0afa09259f 100644
--- a/dom/base/nsDOMWindowUtils.cpp
+++ b/dom/base/nsDOMWindowUtils.cpp
@@ -647,7 +647,7 @@ nsDOMWindowUtils::SendMouseEventCommon(const nsAString& aType,
   event.inputSource = aInputSourceArg;
   event.clickCount = aClickCount;
   event.time = PR_IntervalNow();
-  event.flags |= NS_EVENT_FLAG_SYNTHETIC_TEST_EVENT;
+  event.mFlags.mIsSynthesizedForTests = true;
 
   nsPresContext* presContext = GetPresContext();
   if (!presContext)
@@ -1654,7 +1654,7 @@ nsDOMWindowUtils::SendCompositionEvent(const nsAString& aType,
     compositionEvent.data = aData;
   }
 
-  compositionEvent.flags |= NS_EVENT_FLAG_SYNTHETIC_TEST_EVENT;
+  compositionEvent.mFlags.mIsSynthesizedForTests = true;
 
   nsEventStatus status;
   nsresult rv = widget->DispatchEvent(&compositionEvent, status);
@@ -1733,7 +1733,7 @@ nsDOMWindowUtils::SendTextEvent(const nsAString& aCompositionString,
   textEvent.rangeCount = textRanges.Length();
   textEvent.rangeArray = textRanges.Elements();
 
-  textEvent.flags |= NS_EVENT_FLAG_SYNTHETIC_TEST_EVENT;
+  textEvent.mFlags.mIsSynthesizedForTests = true;
 
   nsEventStatus status;
   nsresult rv = widget->DispatchEvent(&textEvent, status);
diff --git a/layout/base/nsPresShell.cpp b/layout/base/nsPresShell.cpp
index c271c454ba2c..340d6c2d9c55 100644
--- a/layout/base/nsPresShell.cpp
+++ b/layout/base/nsPresShell.cpp
@@ -5700,8 +5700,8 @@ PresShell::HandleEvent(nsIFrame        *aFrame,
   NS_ASSERTION(aFrame, "null frame");
 
   if (mIsDestroying ||
-      (sDisableNonTestMouseEvents && NS_IS_MOUSE_EVENT(aEvent) &&
-       !(aEvent->flags & NS_EVENT_FLAG_SYNTHETIC_TEST_EVENT))) {
+      (sDisableNonTestMouseEvents && !aEvent->mFlags.mIsSynthesizedForTests &&
+       NS_IS_MOUSE_EVENT(aEvent))) {
     return NS_OK;
   }
 
diff --git a/widget/nsGUIEvent.h b/widget/nsGUIEvent.h
index b9d2e4f9d3c7..672a98d6261b 100644
--- a/widget/nsGUIEvent.h
+++ b/widget/nsGUIEvent.h
@@ -114,11 +114,6 @@ enum nsEventStructType {
 #define NS_PRIV_EVENT_FLAG_SCRIPT         0x0080
 #define NS_EVENT_FLAG_NO_CONTENT_DISPATCH 0x0100
 #define NS_EVENT_FLAG_SYSTEM_EVENT        0x0200
-// When an event is synthesized for testing, this flag will be set.
-// Note that this is currently used only with mouse events, because this
-// flag is not needed on other events now.  It could be added to other
-// events.
-#define NS_EVENT_FLAG_SYNTHETIC_TEST_EVENT 0x1000
 
 // Use this flag if the event should be dispatched only to chrome.
 #define NS_EVENT_FLAG_ONLY_CHROME_DISPATCH 0x2000
@@ -541,6 +536,9 @@ public:
   // If mDispatchedAtLeastOnce is true, the event has been dispatched
   // as a DOM event and the dispatch has been completed.
   bool    mDispatchedAtLeastOnce : 1;
+  // If mIsSynthesizedForTests is true, the event has been synthesized for
+  // automated tests or something hacky approach of an add-on.
+  bool    mIsSynthesizedForTests : 1;
 
   // If the event is being handled in target phase, returns true.
   bool InTargetPhase() const