Firecrackers can be useful in maps that have ledges, especially Kitchen. I once had 2 Bumblebees and a ton of maggots fall down from the ledges to their deaths. However, you should not use this turret in any other maps that do not have ledges.
The AoE is tremendously large and can knock Centipedes. Unfortunately, it takes a while before the rockets land on their target, and the aim is atrocious, meaning the enemies could have moved by then and it can knock food eaters toward your base, helping them!
Upgrading is very important as it increases damage and firing speed. Having a Scouter is best, too, since they will be able to fire enemies nowhere near the base.
Kitchen and Restaurant are most preferable to use these because of tight spaces, but Office, Pond and Attic are also acceptable. Don't overuse them like what the picture shows. More than 1 Firecracker is not recommended if you just want to knockbaack some enemies.