If I’m fired or quit my current job, can my employer tell me that I can’t work within 30 miles of the business?

If you signed it, it is legal--at least in part, in the case of you quitting or resigning. A non-competition agreement is a contract in which you agree to not compete with your employer within a certain geographic radius, for a certain time, in exchange for being offered (or allowed to keep) a job. They are generally legal, since people can voluntarily agree (and even if you were worried about being fired, it is "voluntary" in the law's eyes, since you had the option to not sign and seek other employment) to all kinds of things, even things that are, frankly, bad ideas.
But there are limitations, based in part on the law of contracts, and partly on the fact that while non-competes are legal, they are not liked or favored, and the courts do not want to completely preclude or prevent someone from making a living:
1) They only apply if you quit or resign, not if you are fired: if you are fired, the company is taking away it's end of the bargin (making a job available to you), and if it does that, you are not longer held to your end. In additition, the courts consider it inequitable or unfair to allow an employer to prevent you from competing--then fire you, living you with no means of support. So only if you voluntarily leave employment should this be binding.
(Exception: if you received something in addition to being allowed to keep your job in exchange for signing, like a bonus, debt forgiveness [if you had borrowed money], or stock, it will be enforceable even if you are terminated, since in this event, even if you lose your job, you still received something of value in exchange for your promise to not compete.)
2) It will only apply to competitive employers or positions or jobs, since the purpose is to protect your employer from you using knowledge or connections gained at it from competing with it. So no court will stop you from any/all jobs, or jobs which in no way compete with your employer--but they will enforce the agreement in regards to competitive jobs.
3) They will not last forever, just for a reasonable time, so you don't compete right away. For most employees, courts will enfore an agreement lasting between 6 and 12 months.