Jackson reloads in almost half the time (5.2 vs 10 seconds) and does 240 damage with AP rounds that also buff its pen and accuracy.
It's also cheaper than the firefly, has better mobility, turret rotation and vehicle crews/super glue.
Back in the alpha the firefly used to have pretty good armour and a 6 second reload, also the rockets could target infantry and decrew paks. relic in all their wise wisdom nerfed all three rather than just the tulips which were really the only problem (it still used to lose 1vs1 vs a panther).
Now brits have a decent but massively overpriced TD that dies to a single puma or StuG. Hell I saw a replay earlier of a guy circle strafing a firefly WITH his tiger