ANSWER: Yes. You can be fired from your job as the result of a medical condition that precludes you from performing one of the critical elements of your job, and yet you may not be eligible for disability retirement because you do not meet one of the other requirements.
Example: Dagwood, a CSRS employee with four years tenure in federal service, was completely paralyzed from a skiing accident. The agency can legally fire Dagwood on grounds that he can no longer perform one or more of the critical elements of his job. Unfortunately, Dagwood is ineligible for disability retirement because he does not have the requisite five years tenure required by CSRS.