I think Firefly looks pretty cool. An in-battle transformation would have been better but I'm also happy with Sam all the time, and the ult looks great with the translucent wings. I'm inclined to get her, maybe her cone, and then Ruan Mei on a rerun who I already regret not having.
Firefly not generating energy during her ult means you either need to find 120 energy somewhere, get a lot of action advance, or suck it up and take 2 turns charging. With E1 there are probably some very complicated but strong speed tuned setups with Bronya. Maybe you do a basic attack during Firefly's ult and then meet up right at the end to advance her first charge. Asta could ult off-sync with Firefly to accelerate her slow turns.
But her natural team, as people have said, looks like an upcoming support character and Ruan Mei with a sustain. Running the numbers I actually think that Firefly can probably get away without a BE rope and still hit 360%. The buffs from support characters could plausibly give +100% and she can get ~213 from her kit, cone, and relic set bonuses. That would let you take an Atk rope and ease some of the substat pressure, or keep the rope and ignore BE substats to just build crit.