Actually, thinking about it there is a good general tip against Harley - do not let db3~1 go unpunished.
Inexperienced and/or disrespectful HQ players have a tendency to throw it out and hope it'll net them a juicy full combo. But it's -10 on block, so make sure you hit them with a powerful punish. And d1~db3 can be easily poked out of with a bit of practice as well.
Onto more specific tips, you're in luck as a Firestorm main because her b2 has major whiffing issues on characters with floating hitboxes. This happens to be her best footsie tool, as her only other ranged normals are atrociously low on start-up, and her mids in general are poor. This means if you space outside of d1 range, Firestorm can easily out-footsie her with safe pokes.
At fullscreen, it's a mixed bag - FIrestorm bf1 obviously out-damages her df1, but its recovery is so fast that occasionally she can land a hit and only have to block the incoming projectile or potentially even having it whiff over her. You probably want to stay within Molten Trap range if you can, and not let her set up anything with trait (though reckless trait calls can easily be interrupted with bf1 anyway lol)
Harley's j2 can be mildly annoying, but in all fairness you're better off just blocking it and then immediately switching to low block after - cross-up spacing on it is comparable to Batman's j2. While Harley does have an overhead option in f2, it's 26 frames, so with some practice you can block it on reaction. No one will judge if you do get hit by it mind - it's happened to everyone at least once! You can always pushblock to avoid the possibility if you're that worried about it.
Honestly I think Firestorm's probably got the edge here.
