"You'd be like a firefly" A literal firefly with a glowing butt. Gokudera chuckles, even if it could hurt, it'd still be terribly amusing. Yamamoto saying it might be worth it gets Gokudera to chuckle all the more, and he's got a very light expression on his face. Why the idea of Yamamoto's pants ripping or on fire is so funny is probably because Gokudera's not a very nice person.

"If I pet them? No. If someone else does... Who the fuck would I let touch my head, huh?" Gokudera looks unimpressed by the notion. He may not be a cat, but he has the attitude similar to one. No one is allowed to touch him without him getting angry about it to some degree.