Fireblossom Seeds can be found in hell when harvesting Fireblossom. Only a blooming Fireblossom plant will yield seeds when harvested, and the only way to make a Fireblossom plant bloom is to submerge the Plant with lava. When growing the fire blossom in a Clay Pot, you still need to submerge it with Lava. The easiest way to harvest your initial seeds out in The Underworld is to place blocks two high on either side of the plant and then pour a Lava Bucket into the gap, thus submerging the Fireblossom in lava. Now, when you harvest the Fireblossom, it will also drop Fireblossom Seeds.