Firmware updates are automatic. They happen when Amazom pushes them to your device. In order for it to not update, you could just keep off wifi and 3g like I do. Even so, when you turn on wifi or 3g it could update right then and when you put it to sleep it will update. Amazon pushes them at different times apparently like cell phone firmware updates because while others on this forum have gotten 3.2.1 I haven't. I am guessing they do this to prevent 3g from becoming overloaded.
With jailbreak, there is supposed to be a feature that does not allow you to use amazon updates until you uninstall jailbreak, but I am pretty sure that will not work with us because none of the other features worked for me when I used jailbreak.