diff --git a/layout/forms/nsListControlFrame.cpp b/layout/forms/nsListControlFrame.cpp
index d30e80f49831..e744e12b2a50 100644
--- a/layout/forms/nsListControlFrame.cpp
+++ b/layout/forms/nsListControlFrame.cpp
@@ -2108,10 +2108,25 @@ nsListControlFrame::KeyDown(nsIDOMEvent* aKeyEvent)
   MOZ_ASSERT(keyEvent,
     "DOM event must have WidgetKeyboardEvent for its internal event");
 
-  if (keyEvent->IsAlt()) {
-    if (keyEvent->mKeyCode == NS_VK_UP || keyEvent->mKeyCode == NS_VK_DOWN) {
-      DropDownToggleKey(aKeyEvent);
+  bool dropDownMenuOnUpDown;
+  bool dropDownMenuOnSpace;
+#ifdef XP_MACOSX
+  dropDownMenuOnUpDown = IsInDropDownMode() && !mComboboxFrame->IsDroppedDown();
+  dropDownMenuOnSpace = !keyEvent->IsAlt() && !keyEvent->IsControl() &&
+    !keyEvent->IsMeta();
+#else
+  dropDownMenuOnUpDown = keyEvent->IsAlt();
+  dropDownMenuOnSpace = IsInDropDownMode() && !mComboboxFrame->IsDroppedDown();
+#endif
+  if ((dropDownMenuOnUpDown &&
+       (keyEvent->mKeyCode == NS_VK_UP || keyEvent->mKeyCode == NS_VK_DOWN)) ||
+      (dropDownMenuOnSpace && keyEvent->mKeyCode == NS_VK_SPACE)) {
+    DropDownToggleKey(aKeyEvent);
+    if (keyEvent->DefaultPrevented()) {
+      return NS_OK;
     }
+  }
+  if (keyEvent->IsAlt()) {
     return NS_OK;
   }