Can I get fired while on temporary disability?
Business with less then 10 employees.
Yes. Being on disability does not prevent one from being fired, laid off, terminated, etc. for any valid reason not related to the disability--i.e. you can't be fired for being disabled or going out on disability, but if there was a restructuring, an RIF, or you were fired owing to poor performance or a conflict with a boss, that can go ahead. Normally, companies will wait until someone is back from disability before laying them off, but it is not actually prohibited, as along as the cause was not itself related to the disability.
It depends. If you were fired solely for the reason that you are on medical leave then, no, it probably is not legal. I say probably because if this leave has been for an extended period (in CA, for example, 180 days or more), your employer has the right to re-place you. However, if you were laid because of other work-related reasons then, yes, it is legal. Being out on medical leave does not automatically mean that you can't be terminated. In fact, since most employment relationships are "at-will", there really doesn't have to be any reason at all.
If, however, if you feel that you have been wrongfully terminated, you can file a complaint with your state's Department of Labor. You may also want to discuss your situation with an attorney who specializes in employment law.