The protections listed above are fairly limited, so if you are concerned that your employer will retaliate against you for what you're saying, or you would just prefer to keep it private, the safest bet is to blog anonymously or to restrict access to your blog.
Where does dating a coworker (or asking someone out) land on the sexual misconduct spectrum?
I recently began dating someone in another department. For more information, see our site's at-will employment page.
So if the reason for your termination is not illegal under the laws of your state, then yes, your employer can fire you for what you do on your own time, outside of work.
If you value your job and truly want to quit smoking, you may want to take advantage of these additional incentives, or you may want to look for a new job without such restrictive policies.
Generally, an employer can fire you for having a personal website or blog that it deems inappropriate, with very limited exceptions.