diff --git a/layout/xul/base/public/nsXULPopupManager.h b/layout/xul/base/public/nsXULPopupManager.h
index 6d7155f7bb54..5045c9db46ae 100644
--- a/layout/xul/base/public/nsXULPopupManager.h
+++ b/layout/xul/base/public/nsXULPopupManager.h
@@ -767,6 +767,9 @@ protected:
   // root prescontext's root frame.
   nsIntPoint mCachedMousePoint;
 
+  // cached modifiers
+  PRInt8 mCachedModifiers;
+
   // set to the currently active menu bar, if any
   nsMenuBarFrame* mActiveMenuBar;
 
diff --git a/layout/xul/base/src/nsXULPopupManager.cpp b/layout/xul/base/src/nsXULPopupManager.cpp
index 788cad55f4f9..88c2278ad6ed 100644
--- a/layout/xul/base/src/nsXULPopupManager.cpp
+++ b/layout/xul/base/src/nsXULPopupManager.cpp
@@ -73,6 +73,11 @@
 #include "nsIObserverService.h"
 #include "mozilla/Services.h"
 
+#define FLAG_ALT        0x01
+#define FLAG_CONTROL    0x02
+#define FLAG_SHIFT      0x04
+#define FLAG_META       0x08
+
 const nsNavigationDirection DirectionFromKeyCodeTable[2][6] = {
   {
     eNavigationDirection_Last,   // NS_VK_END
@@ -140,6 +145,7 @@ NS_IMPL_ISUPPORTS4(nsXULPopupManager,
 nsXULPopupManager::nsXULPopupManager() :
   mRangeOffset(0),
   mCachedMousePoint(0, 0),
+  mCachedModifiers(0),
   mActiveMenuBar(nsnull),
   mPopups(nsnull),
   mNoHidePanels(nsnull),
@@ -462,6 +468,8 @@ nsXULPopupManager::InitTriggerEvent(nsIDOMEvent* aEvent, nsIContent* aPopup,
     }
   }
 
+  mCachedModifiers = 0;
+
   nsCOMPtr<nsIDOMNSUIEvent> uiEvent = do_QueryInterface(aEvent);
   if (uiEvent) {
     uiEvent->GetRangeParent(getter_AddRefs(mRangeParent));
@@ -475,6 +483,22 @@ nsXULPopupManager::InitTriggerEvent(nsIDOMEvent* aEvent, nsIContent* aPopup,
       nsEvent* event;
       event = privateEvent->GetInternalNSEvent();
       if (event) {
+        if (event->eventStructType == NS_MOUSE_EVENT ||
+            event->eventStructType == NS_KEY_EVENT) {
+          nsInputEvent* inputEvent = static_cast<nsInputEvent*>(event);
+          if (inputEvent->isAlt) {
+            mCachedModifiers |= FLAG_ALT;
+          }
+          if (inputEvent->isControl) {
+            mCachedModifiers |= FLAG_CONTROL;
+          }
+          if (inputEvent->isShift) {
+            mCachedModifiers |= FLAG_SHIFT;
+          }
+          if (inputEvent->isMeta) {
+            mCachedModifiers |= FLAG_META;
+          }
+        }
         nsIDocument* doc = aPopup->GetCurrentDoc();
         if (doc) {
           nsIPresShell* presShell = doc->GetShell();
@@ -1187,10 +1211,19 @@ nsXULPopupManager::FirePopupShowingEvent(nsIContent* aPopup,
   }
 
   event.refPoint = mCachedMousePoint;
+
+  event.isAlt = !!(mCachedModifiers & FLAG_ALT);
+  event.isControl = !!(mCachedModifiers & FLAG_CONTROL);
+  event.isShift = !!(mCachedModifiers & FLAG_SHIFT);
+  event.isMeta = !!(mCachedModifiers & FLAG_META);
+
   nsEventDispatcher::Dispatch(popup, presContext, &event, nsnull, &status);
+
   mCachedMousePoint = nsIntPoint(0, 0);
   mOpeningPopup = nsnull;
 
+  mCachedModifiers = 0;
+
   // if a panel, blur whatever has focus so that the panel can take the focus.
   // This is done after the popupshowing event in case that event is cancelled.
   // Using noautofocus="true" will disable this behaviour, which is needed for
diff --git a/toolkit/content/tests/chrome/popup_trigger.js b/toolkit/content/tests/chrome/popup_trigger.js
index 97b37b8b911e..3c0e4af67831 100644
--- a/toolkit/content/tests/chrome/popup_trigger.js
+++ b/toolkit/content/tests/chrome/popup_trigger.js
@@ -3,6 +3,22 @@ var gTrigger = null;
 var gIsMenu = false;
 var gScreenX = -1, gScreenY = -1;
 var gCachedEvent = null;
+var gCachedEvent2 = null;
+
+function cacheEvent(modifiers)
+{
+  var cachedEvent = null;
+
+  var mouseFn = function(event) {
+    cachedEvent = event;
+  }
+
+  window.addEventListener("mousedown", mouseFn, false);
+  synthesizeMouse(document.documentElement, 0, 0, modifiers);
+  window.removeEventListener("mousedown", mouseFn, false);
+
+  return cachedEvent;
+}
 
 function runTests()
 {
@@ -15,17 +31,13 @@ function runTests()
 
   gIsMenu = gTrigger.boxObject instanceof Components.interfaces.nsIMenuBoxObject;
 
-  var mouseFn = function(event) {
-    gScreenX = event.screenX;
-    gScreenY = event.screenY;
-    // cache the event so that we can use it in calls to openPopup
-    gCachedEvent = event;
-  }
+  // a hacky way to get the screen position of the document. Cache the event
+  // so that we can use it in calls to openPopup.
+  gCachedEvent = cacheEvent({ shiftKey: true });
+  gScreenX = gCachedEvent.screenX;
+  gScreenY = gCachedEvent.screenY;
+  gCachedEvent2 = cacheEvent({ altKey: true, ctrlKey: true, shiftKey: true, metaKey: true });
 
-  // a hacky way to get the screen position of the document
-  window.addEventListener("mousedown", mouseFn, false);
-  synthesizeMouse(document.documentElement, 0, 0, { });
-  window.removeEventListener("mousedown", mouseFn, false);
   startPopupTests(popupTests);
 }
 
@@ -254,7 +266,7 @@ var popupTests = [
   // can be used to override the popup's position. This test also passes an
   // event to openPopup to check the trigger node.
   testname: "open popup anchored with override",
-  events: [ "popupshowing thepopup", "popupshown thepopup" ],
+  events: [ "popupshowing thepopup 0010", "popupshown thepopup" ],
   test: function(testname, step) {
     // attribute overrides the position passed in
     gMenuPopup.setAttribute("position", "end_after");
@@ -403,7 +415,7 @@ var popupTests = [
 },
 {
   testname: "open context popup at screen",
-  events: [ "popupshowing thepopup", "popupshown thepopup" ],
+  events: [ "popupshowing thepopup 0010", "popupshown thepopup" ],
   test: function(testname, step) {
     gExpectedTriggerNode = gCachedEvent.target;
     gMenuPopup.openPopupAtScreen(gScreenX + 8, gScreenY + 16, true, gCachedEvent);
@@ -558,6 +570,14 @@ var popupTests = [
     checkActive(gMenuPopup, "", testname);
   }
 },
+{
+  testname: "open context popup at screen with all modifiers set",
+  events: [ "popupshowing thepopup 1111", "popupshown thepopup" ],
+  autohide: "thepopup",
+  test: function(testname, step) {
+    gMenuPopup.openPopupAtScreen(gScreenX + 8, gScreenY + 16, true, gCachedEvent2);
+  }
+},
 {
   testname: "open popup with open property",
   events: [ "popupshowing thepopup", "popupshown thepopup" ],
diff --git a/toolkit/content/tests/widgets/popup_shared.js b/toolkit/content/tests/widgets/popup_shared.js
index d5394d72bfea..5f4d0de56e3c 100644
--- a/toolkit/content/tests/widgets/popup_shared.js
+++ b/toolkit/content/tests/widgets/popup_shared.js
@@ -133,6 +133,16 @@ function eventOccurred(event)
       matches = eventitem[0] == event.type && eventitem[1] == event.target.id;
     }
 
+    var modifiersMask = eventitem[2];
+    if (modifiersMask) {
+      var m = "";
+      m += event.altKey ? '1' : '0';
+      m += event.ctrlKey ? '1' : '0';
+      m += event.shiftKey ? '1' : '0';
+      m += event.metaKey ? '1' : '0';
+      is(m, modifiersMask, test.testname + " modifiers mask matches");
+    }
+
     var expectedState;
     switch (event.type) {
       case "popupshowing": expectedState = "showing"; break;