It's hard for me to believe that it would lower your dps by any significant amount just by waiting the split second it takes in-between Fire1 casts.
But that aside, sometimes Fire Starter will proc twice back to back. So you run the risk of wasting one if you continue casting just like you say. So that's why I think it's best just to wait a tiny bit to be sure.
Agree with you about thunder. The proc on that thing is way too rare. It needs some serious love.