The argument I've seen and heard many times is that someone who was been fired before is likely to get fired again. Also applies to medical conditions. You don't want an employee who might get sick and not be able to work.

oh for sure there are examples where people abuse the employment relationship - those who churn jobs simply looking for severance/unemployment, chronic medical issues or absences which lead to very low output, etc. and its understandable that a manager would try to weed those out.

that can also lead to many false positives, IMO. personal example, in my first job out of college my manager and i did not get along. i assume they wanted to fire me and began to build a case against me, emailing HR every morning i was not at my desk by 8:30 AM. i was naïve and the HR rep basically told me i should quit because my manager had "evidence". so i did, and it took a LONG time to find another job because of the "Scarlett Letter".

it then becomes a self fulfilling prophecy since the longer you're without a job the more suspicious it becomes. it all just feels very classically corporate to me, from a time when people stayed in one city/town for a long time and if you had a gap in the resume it was automatically detrimental to you because all the good people already have jobs.

it's illegal to ask about or discriminate on medical conditions though, unless there's a strong, concrete reason to ask - e.g. you don't have to hire someone with uncontrolled epilepsy to be a bus driver, but you can't refuse to hire a receptionist because her lupus might make her call in sick occasionally.

I'm terrified of all this, since I started using a wheelchair. a friend told me she did 10 interviews with no offers, when they were in person, but got a great offer when she interviewed on Zoom. my current job values me a lot, but do I stay there forever? what if I try to switch jobs and nobody wants to hire the cripple?

Are you a developer? There's so much remote work now that you'll absolutely be able to find work. I can't speak to the (illegal) nature of discriminating against you for an in-office developer job though.