Add Firefox support bug link to customizable-select.json (#7486)

Author: Schweinepriester

features-json/customizable-select.json

@@ -16,6 +16,11 @@
       "url":"https://developer.chrome.com/blog/rfc-customizable-select",
       "title":"Chrome's request for feedback on latest proposal"
     }
+    ,
+    {
+      "url":"https://bugzilla.mozilla.org/show_bug.cgi?id=1944403",
+      "title":"Firefox support bug"
+    }
   ],
   "bugs":[