Fool, if your boss fires you, it is almost always after you have had multiple discussions with him/her regarding a shortcoming in your performance or the quality of your work.
During those earlier discussions you will amost always be given the opportunity to be heard. And there is ample opportunity to discuss the shortcomings of your boss and how an improvement in his shortcomings will help you to improve yourself in your own problem areas.
As one who has both fired others, and has been fired himself, you could say I've been on both ends of the stick. Being fired very rarely has anything whatsoever to do with a knock on one's character. Firing (as opposed to being a victim of a cutback) almost always is about a problem with production or quality of work.
So stating that I simply don't have enough knowledge or prior training to "get the job done" is in no way equivalent to a character attack.