If the goods are unwanted and unopened you can return them without question at your cost up to 14 days from the delivery date. Contact the Fireworks support team through the support tab in your order page and they will help you.
Is your item faulty? You have 30 days from the date of delivery to alert the Fireworks Team so they can arrange a replacement or refund for you. Please do help them out with as many details of the fault as possible. Including photos if you can.
If the item is outside the 30 days the retailer/manufacturer is afforded the chance to inspect and repair or replace, whichever is more viable. (This is outlined in the Consumer Rights Act too, details here). Fireworks, as the retailer, can contact the manufacturer on your behalf however a solution is reached significantly quicker if you contact the merchant directly.
If the item is showing evidence of damage or the fault is as a result of wear and tear the return/repair/replacement may be rejected.
If you have any questions about the return process get in touch with them through the support tab in the order page.