Can I be fired because I smoke cigarettes?

It depends on the laws of your state. 

In every state, employers are free to ban smoking at work. Some are required to do so by law; others choose to have a smoke-free workplace to protect the safety and health of employees and customers. An employee who violates a workplace smoking ban (for example, by smoking in a smoke-free building or non-smoking area) can be disciplined or terminated in any state. 

But what about employees who smoke on their own time? In some states, employers may not discriminate against smokers. Some states go even further and prohibit employers from making job decisions based on an employee's lawful, off-duty conduct, whether it involves smoking cigarettes or singing karaoke. In these states, an employer who fires an employee for off-duty smoking can be sued for wrongful termination. 

However, in states that don't have this type of law, employers are generally free to refuse to hire smokers or terminate smokers from the company. To find out your state's rules, see Nolo's Workplace Smoking Laws in Your State.