The Americans with Disabilities Act (ADA) does not prevent an employer from firing a disabled employee so long as the employee is not fired because of the fact that he or she is disabled. An employer certainly may fire an employee who is not meeting performance expectations, disability or not. What the ADA requires is that employers provide reasonable accommodations to help an employee so that he or she can try to meet those expectations. In most cases, the employer’s obligation to provide reasonable accommodations is not triggered until the employee notifies the employer he or she has a disability and requests an accommodation.

The exact details matter, and you have not provided us any details here. What kind of “odd behavior” was involved and did the employee tell the employer he or she had a mental disability prior to being terminated by the employer?