Here's where getting fired is going to matter most. You can be sure you are going to be asked the question "Why did you leave your last job?" Dick Bolles, author of What Color Is Your Parachute, recommends volunteering that you were fired even before the question is asked, then moving on. In Job Interviews for Dummies, Joyce Lain Kennedy gives similar advice, saying, "Keep it brief, keep it honest, and keep it moving." She suggests explaining why (downsizing, merger) if it wasn't your fault. If it was, Kennedy suggests telling the interviewer you learned a lesson and explain how you benefited from the experience. Take the negative and turn it into a positive.
Practice. Take some time to prepare answers to questions about being fired, so you know exactly how you are going to answer. Practice again, so you can respond confidently and without hesitation. The more you say it, the less painful it will be.
Again, don't lie. Most companies check references and background information. If you lie, you are probably going to get caught.
Do not contradict yourself. Tell the truth and have one story and stick to it regardless of how many people are interviewing you. They will compare notes afterwards and you don't want to have told one person one thing and someone else another version.
Do not insult your former boss or your former employer. No employer likes to wonder if you will talk about them that way in the future. Also, don't be angry. Feeling angry after being fired is normal. However, you need to leave that anger at home and not bring it to the interview with you.
As hard as it may be, and it is hard, you need to get over getting fired and move on. You need to be able to convince employers that, regardless of what happened in the past, you are a strong candidate for the position and can do the job. Focusing on the skills and experience you have, rather than the firing, will help sell you to the employer and will help you get the job.