Firstly the accountant breach confidentiality, not you - I assume he is being fired as well, if not why not.
The fact that you have been asked to attend a disciplinary proceeding to be fired would suggest that it was predetermined and that is a breach of process (handy for an unfair dismissal claim). Are you sure that the manager said that you will be fired or that the out come of which could be that your may be fired? There is a big differece here.
Process is really important here - did they send you notice in writing? Have they provided you with evidence?
You can ask for a colleague to be in attendance on your behalf to take notes, if they refuse, this is another mark against them. I would do this if i were in your position, it may help if you do actiually have to go to a tribunal.
Appeal the findings - if they dismiss you or give you a warning which you believe to be unjust, appeal - they should have a process in place for this and should offer you the opportunity when presenting their findings to you.
Start making notes.Write down everything you can remember that was said and by who including the phoone call by the manager. This will help if needed later as memories tend to fail over time. make sure you date the notes when you wrote them. Also see if your colleague will write out his recollection of the incident. Again this will help.
Above all try to remain calm, difficult considering the circumstances, but losing your temper will not help you one bit.