For pure fire Hydra FB Sorc, 3-piece with Orb is usually best option because of -15% enemy fire res. My setup on hardcore fire sorc:
Tal Orb + Armor + Belt (Orb and Armor have -5/+4 Fire Facets)
Spirit Monarch 35fcr
Cta + Spirit on switch
Flickering Flame
Magefist
Treks
Mara's
Soj
Rare Ring FCR/allres
Sunder, Anni, Torch, Fire Skillers, life and res small charms
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Act 2 Prayer Merc has Insight + Cure + Skin of the Flayed One (w/Um)
It's very good, can do pretty much any area of the game. I run Energy Shield with max Telekinesis because .. hardcore and basically unkilleable. In softcore can just max all fire synergies and damage will be 50-80% higher I imagine.
For your question - Flickering Flame is worth it imo for fire Sorc, but Infinity will make farming everything much faster and easier so depends... I would try and go for both if you can, Flickering Flame is much cheaper so get that first probably
I did wind up going with basically this setup. Minus the torch, and infinity. Still working on these. Did ubers like 10x still no storch. 3x ptorchs though zz