What has she got on you that could get you fired?
OK. First things first, get her talking about such threats and record them. I believe Radio Shack and other stores carry equipment by which you can record phone conversations. You can use these recordings against her in a NC court. She doesn't need to be informed that she is being recorded.
After that I guess my reaction, whether good or bad, would be one of two, possibly both:
1.) To head things off at the pass, I would go into work first thing in the am on Monday, and ask to speak with my boss. I'd explain to him although it's not my normal thing to discuss personal issues at work, that I was in the process of a divorce that has suddenly turned nasty. I'd tell him that she has threatened you with attempts to get you fired, and that you are doing your best not to allow any of this to affect your work performance, but that he might expect a call or two from your STBX.
2.) I would explain to my STBX that if she decides to get you fired, that you would go back to court to get the alimony and child support lowered. Since by her own actions she got you fired, a court would would lower those payments since it was an involuntary reduction in income. I'd also add that the court record of that legal proceeding demonstrating her attempts to injure you could be read into evidence at the trial for the final divorce judgement and be used against her. Such vindictiveness would look very badly for her and could cost her much in the long run.