Which fire type do you think was bad in previous gens, but got improved in the newer gens?
I honestly have no idea about this. I've generally stuck to whatever generation-exclusive Fire-type Pokemon in their respective games, and whenever I've used past generation Pokemon in the newer gens, I've never really taken note to any change. If I thought they were bad in previous generations, I doubt I'd catch them in any of the newer ones, and if they were strong in the past generations, chance is I probably wouldn't have noticed any change. That's pretty much where the remakes make a change, but I've only ever played Firered and Leafgreen, and that was kinda the restart to my fandom, so anything I'd learned from the past generation, wasn't something I really could recall at that point.