Unless you have an employment contract, or an employee handbook that limits the reasons for firing in a way that can be used as a contract (and this is very rare), you are an employee at will, which means that your employer can fire you without any reason at all, as long as they are not breaking laws such as those about discrimination. Your question doesn't have anything that suggests anything like that. However, if you have filed a workers compensation claim for the injury, it's possible that you could argue that the firing is in retaliation for that, which is illegal in many states.
For advice that you can rely on, please have all of the facts of your case reviewed by an attorney in your area. One place to find a lawyer is our website, http://attorneypages.com