They seem to open fire on eachother ONLY inside military bases. They don't reproduce this inside a town.
As Opfor sometimes they don't fire on you but if one group fires on you. Another group of the FIA will engage on the hostile group towards you. Sometimes if you shoot some of the FIA troops they will tell you to Cease-fire. This is also the same story for Blufor.
Edit: Sometimes this can be different in any base on the map. Some bases have them silent and reproduce the problem above [Shooting on eachother when they shoot at me] If in a smaller post they will begin firing on eachother straight away.