Did you only float him and then put him in? If so, that was a really insufficient method, and your LFS
guy should be tarred and feathered (I HATE when some LFS
people give good and caring LFS
employees, me ahem, a bad reputation by doing dumb crap or being uneducated like the aforemented). The minimum acclimation for any fish, in my opinion, is to allow temperature to acclimate by letting float for 10-20 minutes, and then to start pouring ~1/4 cup aquarium water into his bag every ~5 minutes after temp is acclimated, until volume is at least double or triple. Anything less than that is a good way to shock and possibly kill a fish. If however that was the method he told you to do, the "cup" method as I call it, then I take back my LFS
I hope your firefish isn't too stunned and pulls out alright.