diff --git a/dom/plugins/ipc/PluginInstanceChild.cpp b/dom/plugins/ipc/PluginInstanceChild.cpp
index 9fa4467afd4..ba1530e832f 100644
--- a/dom/plugins/ipc/PluginInstanceChild.cpp
+++ b/dom/plugins/ipc/PluginInstanceChild.cpp
@@ -2003,7 +2003,9 @@ PluginInstanceChild::AnswerSetPluginFocus()
     // when a button click brings up a full screen window. Since we send
     // this in response to a WM_SETFOCUS event on our parent, the parent
     // should have focus when we receive this. If not, ignore the call.
-    if (::GetFocus() == mPluginWindowHWND || ::GetFocus() != mPluginParentHWND)
+    if (::GetFocus() == mPluginWindowHWND ||
+        ((GetQuirks() & PluginModuleChild::QUIRK_SILVERLIGHT_FOCUS_CHECK_PARENT) &&
+         (::GetFocus() != mPluginParentHWND)))
         return true;
     ::SetFocus(mPluginWindowHWND);
     return true;
diff --git a/dom/plugins/ipc/PluginModuleChild.cpp b/dom/plugins/ipc/PluginModuleChild.cpp
index f437b352dad..42001201ac9 100644
--- a/dom/plugins/ipc/PluginModuleChild.cpp
+++ b/dom/plugins/ipc/PluginModuleChild.cpp
@@ -1910,6 +1910,7 @@ PluginModuleChild::InitQuirksModes(const nsCString& aMimeType)
         mQuirks |= QUIRK_SILVERLIGHT_DEFAULT_TRANSPARENT;
 #ifdef OS_WIN
         mQuirks |= QUIRK_WINLESS_TRACKPOPUP_HOOK;
+        mQuirks |= QUIRK_SILVERLIGHT_FOCUS_CHECK_PARENT;
 #endif
     }
 
diff --git a/dom/plugins/ipc/PluginModuleChild.h b/dom/plugins/ipc/PluginModuleChild.h
index 1c7b555a570..9252eff9644 100644
--- a/dom/plugins/ipc/PluginModuleChild.h
+++ b/dom/plugins/ipc/PluginModuleChild.h
@@ -294,6 +294,10 @@ public:
         // Win: QuickTime steals focus on SetWindow calls even if it's hidden.
         // Avoid calling SetWindow in that case.
         QUIRK_QUICKTIME_AVOID_SETWINDOW                 = 1 << 7,
+        // Win: Check to make sure the parent window has focus before calling
+        // set focus on the child. Addresses a full screen dialog prompt
+        // problem in Silverlight.
+        QUIRK_SILVERLIGHT_FOCUS_CHECK_PARENT            = 1 << 8,
     };
 
     int GetQuirks() { return mQuirks; }