Fusion bombs were among the weapons employed by Vendikar (and, presumably, Eminiar VII) in their war. According to Mea 3, they were "materialized by the enemy over their targets".
In fact, they existed nowhere outside the complex computer models that described the war. Nevertheless, each successful "hit" resulted in massive casualties – and these individuals had twenty-four hours to report for disintegration. (TOS: "A Taste of Armageddon")
These might have been the same as H-bombs, as those are nuclear fusion based weapons.