WatchGuard have released Fireware 12.1 on 18 December 2017. This replaces the previous version of 12.0.2.
Full release notes can be found here. Please read them before upgrading! Your Firebox must be running, XTM v11.7.5, v11.8.4 or v11.9 or higher before upgrading.
Fireware v12.x is not supported on Firebox X 5 Series devices, models 505, 510, 520 or 530.