Just to add....
As far as responding to like an Officer Needs Help call or something in the town you live in, if you get hired by another department, may not always be a good thing.
Unless you deal with that department every day, and know everyone on it, just listening to a scanner and jumping in your car and going to help because you are a cop somewhere else, can turn bad.
You come flying up to a scene where one, they were not expecting you, two, you will probably be out of uniform, and a badge on your belt, or neck, three, may not have any radio contact with them to know any details while enroute, or on scene. Maybe they relayed important info you misses because the agency you work for does not have the same radio system, and they only way you hear them is a scanner at your home. You maybe flying into an ambush, or some running up with your gun out, and get shot by a cop because they don't know you, because they don't work with you, and think you are trying to harm them.
Turn me, if someone, I never met before, showed up with a shirt that just said POLICE on it, in a POV, I would tell them to stand back and wait in there car or whatever, until I had the time to find out who they were, and what they said they were. Granted if I was shot and bleeding, maybe not, but I was trying to not get shot, or it was a stand off or something, last thing I need is someone else there, who I don't even know if they are a real cop, and worry about protecting them.
I have been in LE for about 15 years now, both sworn and non, and I am a deputy in a very rural area (3,500 miles, with 4 street deputies TOTAL) I am the deputy for my area, so off duty or not, I get called out to things. I am given a take home unit for this reason.
I know there are the what if's, but if you live in a city with a dedicated PD or something, let them handle it. You can probably get to a scene a lot fast driving normally, or with your hazards on, then anything else. Or risk getting sued because you were off duty using your vehicle code-3, to another agencies call, and someone crashed because of it. Or worse hit a marked unit, who didn't know to look out for someone responding code as well from a certain area because they knew they were the only unit for 30 miles, and had the green light and you were rolling code and rolled the red.
Like I said tons and tons of what ifs, but leave it to the guys who are on duty, if it is marjor enough they will call you at home to respond. And trust me, there have probably been many and many help calls before, and if it was that major of a concern they will issue a take home.
Don't take anything in a bad way, just don't be too eager to want to go balls out with everything you can. After you do all the stuff you need to, and on the street by yourself you will see that once your home, you want to stay home. If they allow it, just get a SINGLE LED dash light to throw up on your dash, but I am almost willing to bet you will NEVER need it. Remember you wont be on duty, and you probably wont respond more then a few miles, treat it like a VFD courtesy light state (I know OH is not and you can run red and sirens) but use a single LED and drive with care, don't fly to get there, or you wont make it, and you wont do anyone good. Unless you are in a marked unit, or a properly outfitted vehicle, don't drive like it. And I say that as in, you don't need a properly outfitted vehicle, because chances are you may use it in an actual response maybe twice in a career while off duty.
Just some thinking outside of the box. All I am saying is put yourself in an officers shoes. You call Signal-99 Maybe 3 other units already showed up and now this POV comes up decked out, and a guy gets out with a badge you cant really see and a gun or something. You know he is not on your department, you have maybe seen him before, but can't place it. What are you going to do? Stop what your doing, and focus your attention on him thinking maybe he is ambushing you or playing pretend to get close to you to harm you. Or believe what he says, and then trust he has enough skills to not get himself or you killed.
I have helped in situations off duty with officers fighting, but if possible, I always try to make eye contact with the office, and try to wait for him to acknowledge me before I jump in just for the simple fact that I don't want them thinking I am trying to rescue who ever they are fighting with.
Sorry for making that so long. When and IF the time comes, I would sit down with whoever your chief or sheriff or commander is, speak to them about it. THEN if you get the ok, go to whoever is the chief of the town you live in and sit down with them, and talk about it. If they say its okay, then at least you have the backing, and from there you can get to know the local cops.
And I know your probably a firefighter for the town you live in, but if you get hired as a cop, make sure they know it. Because there have been cases, about volley FF getting arrested for impersonating cops on scenes.