Never, never never ever include any negative information in your cover letter, such as the fact that you were fired from a job. Negative information immediately puts your cover letter (and entire application) into the trash.
Your cover letter is a marketing document. Thus, talk only of the great things about you and how you will make a contribution to your future employer. Discuss what you can bring to the employer; discuss your key skills and qualities.
Take some time to go through a wonderful tutorial on cover letters. Go to the Dynamic Cover Letters Tutorial For Developing a Stunningly Effective Cover Letter.
Hiring decision-makers surveyed for the book, Top Notch Executive Resumes identified this as one of their Top 30 Executive Resume Pet Peeves: Resume is not accompanied by a cover letter or cover letter is not targeted to the open position. Not all employers read cover letters (about two-thirds do), but to some of the decision-makers who do read them, cover letters are very important. Your resume should always be accompanied by a cover letter. And given that one of the main functions of a cover letter is to describe how your qualifications match a specific job vacancy, it is pointless to send a boilerplate cover letter that is not tailored to the targeted position.