The Foxes meet defending champions Liverpool on Sunday.
"I played with a lot of the lads from the national team so I was lucky but it's energising (joining Liverpool)," said Heskey, who scored 46 goals for Leicester after starting his career at Filbert Street.
"You are with some of the best players in the world and it can only make you better. He's been excellent, he'll be being pushed and pushed differently.
"But Firmino is still key, he's an integral part of how they play. He's a fantastic player, he may not be all about goals but it's about movement, trickery, guile, playing balls in. We've seen it all.
"I've said it about Michael (Owen) as well. He'd be playing injured but just having Michael there gave others a boost as well because they know they have the talented man on the pitch."