Hi,
In all honesty I don't think earmuffs would really muffle the noise of fireworks enough to make a difference. Something that may work though are the noise-cancelling head phones. You can get them at electronics stores and even Target has them in their electronics department. They range in price and quality - Bose (the best, IMO) has a pair for $350, but you can get lesser quality ones that may work well enough for under $50 at Target or other electronic stores. I really don't know how well they work for fireworks but I have used them on an airplane and they really do cancel the noise pretty well. I do think that the cheaper ones do not work as well though so you may want to check the store return policy if you decide to buy some and try them out.
HTH!
Cheryl