--- a/embed/mozilla/EphyXULAppInfo.cpp	
+++ a/embed/mozilla/EphyXULAppInfo.cpp	
@@ -128,3 +128,12 @@ EphyXULAppInfo::GetXPCOMABI(nsACString & aXPCOMABI)
   aXPCOMABI.Assign (EPHY_HOST_CPU "-gcc3");
   return NS_OK;
 }
+
+/* readonly attribute AUTF8String WidgetToolkit; */
+NS_IMETHODIMP
+EphyXULAppInfo::GetWidgetToolkit(nsACString & aWidgetToolkit)
+{
+  aWidgetToolkit.Assign ("gtk");
+  return NS_OK;
+}
+