An interview panel has two basic rationales for asking this question. First, it's important to determine whether an applicant has given much thought to his career progression. Second, an interviewer wants confirmation that you're a good long-term fit with the department. The ideal answer will focus on a specific career goal, and how you might achieve it. The How To Become A Firefighter In USA website recommends saying something like, "In the next five years I would like to see myself having developed an expertise in fire and rescue. I want to stand out from my peers and demonstrate my passion for my work. I also hope to be promoted so that I may serve as an example for others.” Failure to respond, or take the question seriously, will count against you.