Really good game
The art looks professional, but the controls are a bit of a drawback. I like games with as few keys as possible. The jump should have been the up arrow and the release should have been a double-tap of the fire button, like GammaFoxTBG suggested. The action key can even replace the kick, as the kick doesn't seem to be of much use. (Maybe the kick would be better if it had a longer reach, could damage boxes, and/or could knock the enemies farther away.)
And I may just be too uncoordinated, but there seemed to be a slight lag from when I pressed the jump key to when he actually jumped. Maybe that is intentional or maybe it's my computer, but I'd die alot when I'm trying to jump over acid--half the time, he'd just skid off the edge of the platform.