Try to get a lot more tank decorations and plants, it looks sorta bare right now in that area. Without a doubt they're going to be a pesky when they're breeding, and although firemouths are a tad more relaxed than some other cichlids, they're still territorial. And like all cichlids (Including angels!) they only get worse when they breed. Make sure to get a lot of plants, driftwood, and caves.
It's good to see you're using sand, though, firemouths do love to dig!
It's good to see you're using sand, though, firemouths do love to dig!