Click inside the applet to initialise keyboard controls. Then use the numeric keypad to pilot your ship. 0 or spacebar fires. Note that your craft has Artificial Intelligence (and hence Artificial Stupidity) built into it - it won't fire the lasers if there is nothing to hit in a vertical line upwards from the craft, it also detects which laser to fire on the basis of the object it is firing at. Each cube takes two shots to destroy - one to make it a mine, and the second to destroy it.
Once a cube has been made into a mine it can kill you by landing on you, or by landing on the gameboard in a position that you later walk over. Game ends when there are no more cubes in the air - you will have to reload the page to begin again. If you finish prematurely by dying, you get a message telling you so, and the game will restart automatically.
In addition the firing mechanism obtains a lock onto the object you would have fired at if you'd stayed in the position you were at when you fired. If you move too quickly, and java doesn't have time to catch up with the animation, the Artificial Intelligence on board will fire a crooked shot at the object you had initially locked onto once it has finished moving. Note that all laser shots are in front of all other objects. This is a native characteristic of the laser being fired - ie a light source that can be seen through an opaque, thick object as if it wasn't there (Technology has advanced in silly directions).