Not if you work for a private employer. Like the rest of the federal Constitution, the First Amendment protects all of us from the government, not from private companies. While the First Amendment prohibits the government from telling us what we can say (within reason), it doesn't apply to private employers.
However, other laws may protect bloggers and posters from employer discipline, depending on the subject matter of your posts. For more information, see Nolo's article Fired for Blogging.