Hi and welcome to the Chiari forum.
Have you been retested? The reason I ask, is a blow to the head can cause low lying tonsils to form, and if you did not have symptoms, it is possible the tonsils retracted.....However if, in fact you have Chiari , which is a congenital condition anything at anytime can cause symptoms to flare. You may be able to be a firefighter, and you may not it all depends on how your Chiari will affect you doing that type of activity.
We are all different an d our Chiari will affect each of us differently as may of us also have co-morbid conditions along with Chiari and they all will cause varying symptoms.
See if your local firefighters union will allow you to try out their training....see how you do, but since it is a few years, get retested to see if in fact your Chiari is , there.,..and the reason I say that is because low lying tonsils is acquired Chiari.......not Chiari...and the tonsils may have retracted and no longer an issue.
Please keep us posted on what you find out about your Chiari and your firefighting aspirations. Good Luck!