commentator
I know I beat this drum a lot, but in this situation, I would certainly file for unemployment benefits immediately. They will do an investigation of the situation, and you may be approved, which will give you a lot more credibility with other employers. For one thing, it will require your former employer to give the specific reason why they fired you, and provide the documentation they used, the warnings that were given, the circumstances, etc.
Then, when you go to other interviews, you can say, "Well, after fifteen good years, I ran into a situation where I had a conflict with someone and was terminated. But I applied for and was approved for unemployment because it was determined they did not have a valid reason to terminate me in this situation." Employers are all quite familiar with the unemployment insurance system. If you don't bother to go and file a claim, they will assume that you were guilty of whatever it was and didn't want to fight it.
If the reason for termination was gross misconduct, such as beating another worker with a pipe or stealing something, taking hostages, a dramatic incident of misconduct, you'll just have to put it in the best possible light, though you probably won't be approved for unemployment benefits.
If you were caught up by an attendance policy or a violation of a common workplace rule, you'll need to cover that briefly and positively.
In an interview, don't dwell too much on this issue of why you left the last job. Concentrate instead on the fifteen good years you did have with this employer and the skills you acquired there. If your attendance was good, mention that. If you received any commendations, mention that. You obviously weren't too awful an employee, because you had fifteen years before you were fired.
Don't let the interviewer pick up that you are bitter or angry about what has happened to you. No one wants to work with a bitter or angry employee. They may ask you, "If I speak with your former employer, what will they say about you?" Be prepared to explain a little bit, but don't dwell only on your termination. Keep emphasizing the fifteen years you were a good employee.
The former employer can say pretty much whatever he wants to. However, most of them are pretty cautious in actual practice about badmouthing former employees. Many companies have policies in place where they are only allowed to confirm dates of employment and whether or not they would rehire you. It is up to you to sell yourself to someone else, even if your former company says they wouldn't rehire you.
Incidentally, when completing a company application, you do not have to put "fired for misconduct" under the blank for "reason for leaving". You can put "will discuss during interview" or something similar. Do not leave the space blank.
"Left to find another job" is an honest answer. One of my favorites was the guy who put, "re-captured, returned to prison" as his reason for leaving his most recent job.