Slow your roll and realize that the recruiters are telling you what they tell EVERY swinging johnson that shows up in there. You have 2 years to decide what you wanna do, until then keep your nose clean, stay fit, and keep your grades up. If at 18 you decide the Corps is for you (and it is certainly NOT for everyone, even many active Marines dont enjoy it) then by all means go for it.
Your parent need to get over it. When you are an adult they can either support you or eff off, and in most cases they will support you.
Being a firefighter doesnt mean you wont see combat. There are alot of combat action ribbons floating around EVERY MOS.