You need to figure out:
• were you fired or laid off? The difference: one requires cause, the other does not. He could fire you because you messed up, but laying you off probably requires following some kind of non-discriminatory selection process.
Then there are two types of cases:
• Is it disparate treatment? Do you have evidence that you were treated differently because you were in a protected class? You have to show this in court. The defense has to show why you actually were treated in a non-discriminatory way.
• Is it a case of adverse impact? Your boss may be a racist, sexist pig, but did anything he did on the job cause you harm? If so, did what was his reason? Defense: whatever had an adverse impact on you had to have a legitimate job-related reason.
Here is an example: maybe a cop needs to be able to run an 8-minute mile with some heavy gear, and maybe this is a bit harder for women than for men. There is an adverse impact for women, but there is a job-related reason for it. The employer may have a burden of showing why this is necessary to do the job. Requiring women to run a 7-minute mile while men only need to run an 8-minute mile would definitely be; it makes absolutely no sense for men to get to run a mile more slowly than women. There is an abverse impact element here for the women who don’t get selected.
The disability thing is probably mostly in the adverse impact category. Yes, an employer can require your disability not get in the way of performing your job adequately. Yes, this can have an adverse impact on you, and the employer has the burden of showing why it is necessary. You probably the same right right to your sick days that everyone else has (anything else would be disparate treatment), but your higher use of sick days compared to other employees could be a legitimate factor in a decision to terminate in spite of your disability, but not because of it. That said, there may be special laws protecting the disabled in these kinds of cases that I’m not aware of.