Opera: Seems to trap the z key; Seems to have rendering issues on the aliens and bombs, which seems to be dependent on layout of surrounding blocks for no obvious reason. It almost feels like it is trying to optimise movement and overdoing it.
IE: Seems to have a problem with the vertical positioning where the game area is fixed at the top left of the screen rather than the top of the cell. Also, it doesn't support transparency on png's so you get blocks around the aliens and ships (and konq and Opera don't like GIFs)
Firefox: Doesn't seem to like sending a keydown event when another is already down so you have to key-up z/x before hitting space to fire which is annoying.
Konqueror: Just works (and fast!).