Stay physically fit and perform well in both training sessions and on calls and you should be fine. Accidentally set fire to your own fire station, and your job situation may be slightly more tenuous.
Honestly, your biggest concern is not about being relieved of employment. It's about suffering the debilitating or fatal effects of one of the infernos you are forced to face. If you lose a limb or have first-degree burns over 30% of your body, yeah, you may lose your job, but it may not be the foremost thing on your mind. Work on hanging on mentally, emotionally and physically; if you can handle that trio, your job should still be there for you.