My parents smoke, and I am really worried, none of my other friends parents smoke but mine do. I don't want friends to come round because they can see the cigarette butts. They are lovely people and all, but I'm embarrassed. When I was at school, when the subject smoking comes up I say nothing. I feel awful, I have tried to do something about it, but they just get angry, they want to stop, but I don't they will. I'm scared they will die early.

Just be open about it and tell them how you feel about them smoking.
I think most of us are embarrassed by our parents for one reason or another. No one will judge you for your parent's habits. And if they do, fuck em'.
Have you tried talking to them about it? You're their kid, I'm sure it'll have an effect on them.
I wouldn't think it was something you should be overly embarrassed about though, no.