I once worked in a big company.
I came to understand that HR's #1 priority is to prevent their employer from being sued. Their primary role is to prevent lawsuits. HR does not "work" for employees. They only care about who pays their bills. And that would be the employer.
If you have a poor attendance record and if your work has suffered in the past, then those are things that can be easily documented. If they want you out, then they could create an extremely strong case against you.
Most likely, if the do fire you, then they will probably offer you some sort of severance and agree to act as a reference for you in the future. You would also, most likely, be able to collect unemployment benefits. In return, they will have you sign a piece of paper agreeing you will not sue....
I say... Go to rehab now and then go back to work if it is an option. It is much better for you to be fired by them than it is for you to simply quit.