Went back to it and after a few tries got 71. It seems the best strategy is to fall near the bottom then mash hard so you get a lot of fireballs in a short amount of time, and if you don't mash then there's only so many fireballs you can get without hitting the ceiling. I really like that because it makes the game harder considering you have more to dodge, and mashing is distracting, so it's kind of like an optional "hard mode" that's purely an organic part of the game's design. The game is surprisingly deep for how simple it is and for how you control it with 1 button.
[Edit]
Played some more, this game is really addicting. I think part of why it's great is it has a good mix of short term feedback and longer term, more nebulous feedback form your actions. Short term being "I got hit by a fireball, I suck at dodging" and long term being "the birds are getting in the way, I should have made more balls" or "there are too many balls, I should have made less". I like that the birds occupying the space above and below you isn't entirely in your control, it gives variety to the gameplay sessions but yet you do have SOME control over it because you could have maybe fixed a bad situation by making more fireballs. It's mean, but games could use more meanness, having everything be purely a test of skill or knowledge of the game can get boring fast.
Maybe I'm thinking about it too much, but design-wise I think this game is really smart and it's kinda inspiring to me.