Basically, if you have CF but stay away from smoking then you have a good chance of living an almost normal life.
Smoking causes inflammation in the lungs, which causes lungs damage, which in turn kills most people, eventually. Before that, it makes them slower, less fit, and poorer. (You have seen how expensive smoking is, yes?)
CF causes inflammation of the lungs, which causes lung damage, which in turn kills most of us, eventually.
Smoking AND CF creates HUGE amounts of inflammation, which causes rapid lung damage, which kills the people who smoke a lot more quickly than just CF does. And before it kills them it makes them very, very ill, much sooner than CF might.
If you have CF and you smoke then you will only be able to live a normal life for a very short time. After that it's just hell all the way.
It's your choice. Be sensible, or be foolish. You choose!
Take care, look after yourself,