Quoting Juggernog-aholic:" As long as it's not affecting your work and you don't do it in the office, can they really fire you for that?"
I think it depends on the state, but here - DH's employer will not hire smokers, or allow them. You can get fired for going against their policy, which includes no smoking. Of course, there are people who lie, and they have to get in their car and drive down the block to smoke. It's a very large office with tons of security (because the company itself is huge and only employees are allowed past a certain point - with an ID badge that scans them into the building, and their own security code), so security would notice if you were smoking anywhere near the building.
OP-If it were me, and it was a good job, I would quit smoking and try to keep my job. That's just me though. I would never give up a good employer so I could smoke.
And technically, being a smoker isn't protected under federal discrimination laws (such as discrimination for race, sexual preference, etc) so I believe it may actually be able to discriminate based on smoking in ANY State. I could be wrong on that, but I know it's legal in my State, at the least.