Is there any announcement for Firebox to extend feature set supported with DH 16 ?
Currently at Key-Exchange Security and Perfect Forward Secrecy, for the Diffie-Hellman key exchange Fireware can use DH groups 1, 2, 5, 14, 15, 19, and 20.
There is an open feature request, FBX-7227, which includes adding DH groups 16 and 18. There's no ETA as to when this might be available.
If you'd like to follow this issue, please create a support case and mention FBX-7227 somewhere in the case. The technician can set the case up to follow it for you.
WatchGuard Customer Support