Stat wise, best TD in the game is Jackson, followed by SU85 if you get them to vet 3. Firefly is actually the weakest on vet3.
Both Jackson and SU85 get insane penetration buffs, while firefly dont. On top of that vet 3 firefly takes 12 seconds to deal 480 damage (2 hits), SU-85 takes ~12 seconds to deal 480 damage (3 hits) and Jackson can deal 4 hits in 12 seconds.
Basically Fireflies are only good at alpha striking, after that they are objectively the weakest ones.
-offtopic-ish: I seem to have horrible luck with jacksons, I have seen tightrope missing/bouncing 8 out of 10 fired shots at a king tiger and never liked the jackson again
additionally alpha strike sometimes matters more vs reactive players, the tulips can do wonders sometimes