ContentParent reads the dom.ipc.processPrelaunch.enabled and dom.ipc.processPrelaunch.delayMs preferences at startup when executing ContentParent::StartUp() but does not react to changes to these preferences. Since ContentParent::StartUp() is invoked before Preferences::ResetAndReadUserPrefs() is called the values for these preferences manually entered via about:config have no effect. It is not clear if this was intended behavior or not hence the UNCONFIRMED status. To fix this issue if a change that sets dom.ipc.processPrelaunch.enabled to false is detected one would probably need to invoke sPreallocateAppProcessTask->Cancel() to cancel any pending preallocated processes which might have being scheduled for creation and to clear sPreallocatedAppProcess.