Not sure who it was against, would have been 2005 or 2006 probably and he was on Cleveland. I only remember this because the Padres traded for him right after the season ended.
Edit: there's a Wikipedia page with the list of guys who hit first AB homers. Four players have hit grand slams; Kouzmanoff hit his off Edison Volquez.
I think his brother Bob was a friend of Cosmo Kramer's.