Your supervisor is a waterhead. Was it your supervisor or your sort manager that fired you? PT/FT sups don't have the authority to fire someone in the hub - usually only the sort/division manager can pull the trigger on that sort of thing. It might have been an "off the books" termination where they want you to go home and tremble for a few days before you get a call to come back to work, find out if a termination letter was mailed by the company.
Calling a female co-worker a c*nt isn't insubordination, it's violating the UPS anti-harassment policy and Art. 37, Sec. 1 of the CBA (treating employer/employees with respect).
Also.. one other thing: how far away was she when you allegedly said what you said? For future reference: Let her have her hissy fits and just keep your distance. Hopefully your co-worker is a solid guy and backs you up.