Firstboot has been deprecated and replaced by Initial Setup in Fedora since Fedora 18. In RHEL7 Firstboot is also deprecated but still available to host legacy Firstboot plugins.
firstboot is the program that runs on the first boot of a Fedora or Red Hat Enterprise Linux system that allows you to configure more things than the installer allows.
It also has a mode known as reconfig mode to let you reconfigure user settings that were originally set in the installer. This is generally useful to IHVs shipping hardware to an end user so that the end user can do things such as select their language, set their root password and configure their network. To start this mode, touch /etc/reconfigSys on your system.
firstboot has a modular architecture such that each screen is a separate python module. This allows firstboot to be a framework that requires no knowledge of the underlying modules. The primary benefit of this is that additional steps can be added without having to modify firstboot itself. The Modules page describes how to create a firstboot module.