(Assuming US, dunno about other countries.)
If you're paid for your service on-call, you have Equal Opportunity employment rights. I would start with a gentle reminder. If you're not willing to be out yet, a seemingly joking "maybe I'll argue that I'm transgender" might be called for.
Yes, an employer can require you to cut your hair if long hair impacts your work performance or safety. It is not legal to use hair length to discriminate by gender or against religious practices or anything like that. Basically, the work policies have to be reasonable. (e.g. If you were an actor playing a short-haired character, no luck.)
The wrinkle is if you're an independent contractor (no withholding, you pay your own taxes) you don't have the same protection that an employee would. (I am not a lawyer, but if you can only, reasonably, work for one VFD that strongly argues that you are an employee. You don't have the option of selling your services elsewhere, thus you are entitled to employee status.)
This may have tax implications. Which sucks, but that's America for you. Employees misclassified as ICs are common here, especially in on-call or itinerant positions.