Every bit of support in a strapless bra comes from the band, because there is no strap contribution at all. At FF cup that band has to be genuinely firm, and the cup has to be constructed in multiple sections so it holds its shape without a strap to help it.
What a strapless bra needs at FF cup
Buy it on the tightest hook and wear it in — as the elastic relaxes you move to looser hooks and the bra keeps fitting. If it is comfortable on the loosest hook when new, it is already too big and it will slip. Look for silicone that sits flat against the skin along the top edge and the band, not just a thin strip inside the cup.
Also worth knowing
Many strapless styles at this size convert to multiway, so one bra covers halter, one-shoulder and crossback as well. See multiway bras and the full strapless bra range.