The issue that the Firefly has when facing other Lights is that the slightly slower movement speed doesn't allow for that extra turn during movement without losing a point of your to-hit modifier. A 6/9/x can walk 5 and turn 1 or run 7 and turn 2 facings to get an angled shot, but the FFL either walks straight ahead, runs and makes only a single turn, or loses a point on its modifier. The 4 jump distance won't give you the +3 to match your own penalty for jumping, while a 5 or 6 jumper would get the +3 movement modifier. In a fight between heavier units with low modifiers, +1 taking you from 8 to be hit up to 9 is significant but not monumental, but with Lights getting high modifiers, a +1 taking you from 11 to be hit up to 12 is a 3:1 advantage. The FFL ends up on the losing side in that situation, as a part of its intended role in a light lance. That's a "failed design", in my opinion, although it's still got enough firepower and armor to be "useful" in some roles.
Ideally, it needs one more point of speed, or at the very least, one more jump jet, to make it competitive in a scout or scout-hunter role. Not much use for a scout that can't run away from trouble, or a scout-hunter that can't catch its intended prey. That leaves it either in the role of an under-weight Medium, or searching for unescorted Panthers that it can actually catch and fight. Just about anything else can either run away from it, or it needs to run away from.