Lucky you. This is the only non-browser game I have rated in this jam.
The concept itself was interesting. I liked it quite a lot but as always there are places this could be improved and I think you'll gain more from me pointing out those things (you already know what makes you like this game) :).
Firstly, the level design can be a tad confusing at times. I sometimes ended up in random rooms of fire which didn't seem to have much reason for existing. Perhaps these were here to make the levels feel bigger or to add a tiny bit of exploration? In that case, next time you should try to come up with subtle clues as to which way might be the right way and the dead end areas should have something slightly different about them (blue fire? A tree? A joke of some kind?) just to make it feel a little more interesting :)
The other main thing would be that this game needs a few more mechanics. Lava? Jumping flames? (see mario) A second box summoned with z in certain situations? Different types of boxes in different areas? (wood box that burns? metal box which conducts electricity? Plastic box which floats in water? Spiked block which can kill enemies? That kinda thing) Just a few other things to make it feel less like an endless hallway of the same thing over and over. Adding things like this could introduce a slight puzzle aspect.
Lastly, the mechanic of summoning the box. Sometimes it is summoned on the left. Sometimes on the right. Whilst I can imagine how this came to be, at times it almost seems random it it's by far the most frustrating aspect of the game. A part of me kept wishing it would spawn below me if I jumped. I kept trying even though I was already aware it wouldn't work. Not quite sure why, but it felt like that's where the box should naturally appear. Maybe you could add that in? It gives a great deal more reliability to where the box will appear and as this would happen only when the player is jumping, you'd never have to worry about it spawning in a wall or something.
I'd love to see you develop this further. It has potential :)