Some interview questions are looking for indications of problems you may have had in the past. Never lie about it if you were fired from a previous job, since such information is often uncovered before you are hired. It's best to give an honest answer that minimizes any problems you may have had. When asked "Why did you leave your last job?" you could answer that you and your old boss did not see eye to eye, and then move on to how eager you are to gain a fresh start. Never badmouth a previous employer or co-workers.