Think you are confusing the terms so here is what they are.
mental illness
Any of various psychiatric conditions, usually characterized by impairment of an individual's normal cognitive, emotional, or behavioral functioning, and caused by physiological or psychosocial factors. Also called mental disease, mental disorder.
disability
1. inability to function normally, physically or mentally; incapacity.
2. anything that causes disability.
3. as defined by the federal government: ?inability to engage in any substantial gainful activity by reason of any medically determinable physical or mental impairment which can be expected to last or has lasted for a continuous period of not less than 12 months.?
developmental disability
a substantial handicap of indefinite duration, with onset before the age of 18 years, such as mental retardation, autism, cerebral palsy, epilepsy, or other neuropathy.
So in a nutshell people how have a Mental illness I don't support carrying firearms.