Well you're 18 now and you ARE and adult, regardless of if your parents treat you that way or not. What are your future plans at this point? Do you want to get into the work force and continue towards your goals of getting on the fire dept. or do you plan to continue your education, and if so, are your parents there to support you finacially and give you enough freedom to do it? If they are, you may well be better off playing their game a little longer.
It doesn't sound like that's your parents plan though. They may tell you that you can't move out because you have nothing and no money, but that's where a lot of people start. At some point, that may be your only option. I wouldn't sit around waiting to get on the fire dept. Around here anyway, getting on the fire dept is a difficult thing to do and takes a long time. I'm not saying give up on it, but you'll likely need more freedom to make it a reality and you can't put your life on hold waiting for it to happen.
Talk to people about feelings of depression. A lot of people would feel the same way in your situation. It gets better, way better and your parents are now grasping at straws to control you. It can't and won't last forever, but when and how that day comes will be up to you.
Good luck