Yeah.. The last Amazon update.. 5.0.5.1 blocked firestarter..
I never used it anyway.. But you should now see the kodi icon on your amazon home screen.. for easy access..

Did you install a cache clearner.. ?
Its paramount that you install that.. or something like that.. to keep your stick running smooth..
Most people thing just install kodi.. and off you go.. Thats not the case.. Cache has to be cleaned often..
I set it to clear cache every hour.. That just keep the stick running sweet..