Sometimes, you might be willing to give a fired employee a positive reference. After all, an employee who wasn't a good fit at your business might do well elsewhere. If you can say positive things about the employee, then say them.
If, however, you are not comfortable giving a positive reference, then you shouldn't. In such a situation, less is better than more. When prospective employers call, tell them that you can only confirm dates of employment and job responsibilities and no more. You must take care not to "trash" the employee to a prospective employer, because this will leave you vulnerable to a defamation suit from the former employee.
For more about giving references, see Nolo's article Giving References for Former Employees.