'fine' means different things to different people. Its good to hear that your other inhabitants are not having any issues.
If you really want us to help you figure out why the fish died, you'll need to provide more information that 'everything is fine'. Obviously its not fine if you're loosing fish.
If you know for a fact it was the clown that killed the firefish, and you're looking to have that verified - its entirely possible. Clowns are in the damsel family and can be quite mean. If he's been in the tank a while, I'd introduce new fish with the lights off, you may also want to try re-arranging some of the rocks, or pulling the clown out temporarily to allow the new fish to get established