If I was driving to work and involed in a car accident and I'm still under doctor's care, can an employer terminate you?

In GA, you may be discharged for this reason, or any other reason, even a bad reason. For more info, Google this: Georgia, employment at will. Having a doctor's excuse does not protect your job. The fact that you were on the way to/from work does not change the answer.

An employer can terminate your employment for any reason or no reason at all. Certain exceptions exist (i.e. race, gender, age) and you may also be protected by the ADA in that your employer may be required to provide reasonable accommodations if you have a disability protected by the ADA. Unfortunately for you, being under a doctor's care itself does not prevent your employer from terminating your employment, irrespective of whether you were driving to work. If you were within the scope of your employment at the time of the accident, and your continued injury is now causing you to miss work, then you may also be protected by the Pennsylvania Workers Compensation Act and be entitled to wage loss benefits in addition to medical benefits.

It depends, but probably they cannot fire you if you were not out of work for a short time, or if they are large company. I see this as a disability accommodation issue if you were working there for a while and the problem was that you could not go to work for a while after getting hurt so they fired you while you were out. Usually there is a requirement to accommodate absences from work caused by a physical injury or disease, up to a point. Of course, if the issue is that there were prior absences/tardies and they fired you for missing work that day, although you MIGHT be able to get unemployment benefits, the fact that this last one was something outside of your control may not matter to a wrongful discharge type suit. You should call a lawyer to discuss this in a little more detail.

Yes, you were not on the job at the time, worker's comp is not involved and you cannot perform the duties required of you.

Yes, an employer hires you to work, and if you cannot generally you will be replaced.