If I've been through 2 heart surgeries in the past month and my job is threatening to fire me, is this discrimination?

It may not be discrimination:

1) While there are laws which protect the disabled from being fired, etc. due to their disabilities--

a) Not all medical conditions or surgery are considered "disabilities" for this purpose--and most, in fact, are not. A disability is a condition with a long-term or permanant impact on your ability to do normal life functions; it is not an acute condition, no matter how serious. If a medical condition is not a disability, there is no protection for being fired due to it.

b) Even if your condition would be considered a disability, while you can't be fired simply because you have the condition, you can be fired for other, valid reasons--having a disability does not make you termination proof. So if there are performance issues, absenteeism problems, policy violations, a restructuring, etc., a disabled employee could still be fired.

c) Also, while someone cannot be fired because they are disabled, unless the company and the person are both covered by the Family and Medical Leave Act, there is no guaranteed right to take time off from work--so a disabled employee could be fired if he took too much time off for medical care.

2) If you are older and your firing can be shown to be because you are older (over 40), that may be illegal--but as with disabilities, older employees may still be fired, so long as they are not fired solely due to your age.