One of the very most important is teamwork. As a firefighter, you will do nothing alone. While on duty there will be at least 2, if not 3 or more, other firefighters within ~100 feet of you at all times, 24-hours a day for the length of your shift (typically 24 or 48-hours). During your shift you'll eat together, workout together, clean the station together, etc. It's basically your 2nd family because you'll spend 1/3 of your life with these people. If you're not a teamplayer and you're "THAT GUY" that never takes out the trash, never does the dishes, never carries the heavy tool, always brags, etc. you're going to make everyone around you miserable and nobody will want to work with you.
Another strength you'll need is integrity. People will be inviting you into their homes. They'll be revealing their most private medical secrets to you. People will try to bribe you. And you'll have plenty of opportunities to cut corners. You must be strong enough to resist the temptations to lie, steal, or cheat. If you fail to have integrity, not only will you fail as a firefighter, but you will embarrass and weaken the public's trust in the entire fire service.
The last of the major strengths is hard work. To be great as a firefighter you need to work hard all the time. Work hard at maintaining your physical fitness, work hard at learning your city streets, at memorizing tool locations on the trucks, taking classes, being proactive, moving fast on the fireground, etc. Firefighting is hard work. Embrace it. Don't cut corners or look for the easy way out.
To me, those are the non-negotiables. There are other strengths that I think are important, such as empathy. But to me, these are not absolutely critical because I've met many successful firefighters with varying levels of the following skills. Things I suggest you make strengths are physical fitness, empathy, kindness, resourcefulness, commitment, compassion, foresight, quick decision making, confidence, charisma, communication, resilience, mechanical aptitude, problem solving, creativity, etc.
I hope this gives you a place to start. Please reach out to me if you have any further questions!