The ADA does NOT say that if you have a disabilty, you cannot be fired. A disabled employee can be fired for any reason that a non-disabled employee can be fired for - they simply cannot be fired BECAUSE they have a disability. The ADA is there to level the playing field. It does not require an employer to accept poor performance or behavior that they would not accept in a non-disabled employee - it simply requires that a disabled employee be given the same opportunities to work as a non-disabled person.

Allowing you to make errors and not allowing non-disabled employees to do so, is discriminating against the non-disabled people.

The following is taken directly from the EEOC website. Please note the bolded section.

An employer is not required to lower quality or production standards to make an accommodation; nor is an employer obligated to provide personal use items such as glasses or hearing aids.