I couldn't find any tier list for firefighter, but it would fall into the same category “better in a team/situational."

Justification:

\-Intern (First playable character, I don't have much to say)

\-Nurse (As a cheaper option to buy and stop using internet, it's good, but the space you gain in the long run isn't worth it. It's good as a first purchase if you don't like intern) and paramedic (I don't think the speed boost is bad, or that you're missing out on much considering how cheap it is. It's just ok.)

\-Security (The extra taser is useful, and the camera is also helpful for leaving it in the room and seeing patients' faces directly, instead of switching between two cameras. But, sometimes there's already a secretary and medic at your team), secretary (A very good class if you're playing with two people. You check and the other player responds. It loses its usefulness if there's more than one secretary in the team.), doctor (cheap version of surgeon in a good way. I think it's better than Nurse and Paramedic, so I don't want to put it here, and it works better in a team than playing alone.), firefighter (You prevent your teammates from catching fire, and useful in the slime event. Not so very much alone.)

\-Psychologist (AHA isn't a hard game, but it is chaotic and give you hard times. If you know how to handle the chaos, Psychologist is a VERY good and versatile class.)

\-Head Nurse (Along with Secret Agent, it's a character made to be very good. I don't know what to add, it's really good.)

\-Secret Agent (Same as Head Nurse), Surgeon (A blend of paramedic speed and doctor's sanity. A class you should definitely save coins to get.)

My tier list